boards: st: stm32h7s78_dk: add optional FDCAN support

[bklaric1]

Summary

The STM32H7S78-DK board under Zephyr currently doesn't support FDCAN or better said doesn't have it enabled by default.

This is due to FDCAN1 and FDCAN2 pins already being used by something else.

In the table bellow I will list all the pins that can be used for FDCAN1 and FDCAN2 and what already uses them.
Resources for information this are stm32h7s78_dk-common.dtsi, stm32h7s7l8hxh-pinctrl.dtsi and board's datasheet (Table 22, Pages 132-137).

FDCAN1:

Pin	Functionality	State
PA11	RX	Taken by ltdc
PB8	RX	free
PD0	RX	Taken by uart4
PA12	TX	Taken by ltdc
PB9	TX	Taken by i2c1
PB1	TX	Taken by uart4

FDCAN2:

Pin	Functionality	State
PB5	RX	Taken by timer3
PB12	RX	Taken by ltdc
PB1	TX	free*
PB6	TX	Taken by i2c1
PB13	TX	Taken by ltdc

*Taken by mac in #99296

Describe the solution you'd like

I propose an addition of something like this (of course adapted to this board):

zephyr/boards/waveshare/open103z/waveshare_open103z.dts

Lines 160 to 169 in db0c34d

	&can1 {
	pinctrl-0 = <&can_rx_remap1_pb8 &can_tx_remap1_pb9>;
	pinctrl-names = "default";
	/*
	* make sure CAN and USB are not enabled at the same time
	* because they share interrupts 19, 20 (stm32f103Xb.dtsi)
	* reference: RM0008 rev20 page 205
	*/
	status = "disabled";
	};

to the stm32h7s78_dk-common.dtsi.

This way the user can enable FDCAN without needing to first find the pins in datasheet or pinctrl file, which makes it easier and less complex. The user can add FDCAN support with an overlay like this:

zephyr/samples/drivers/can/counter/boards/waveshare_open103z.overlay

Lines 7 to 15 in f1c2ce4

	/* This board disables can1 by default in favor of USB. */
	&can1 {
	status = "okay";
	};
	&usb {
	status = "disabled";
	};

The main question is what to disable, aka which pins to select for FDCAN1 or FDCAN2.

• ltdc pins are used for display.
• i2c1 pins are used for touch, for the display.
• uart4 is used for console and shell.

Note that I already successfully tested the can/counter sample with FDCAN1 using pa11 and pa12 pins, with display not in use.

Would an addition like this be good? Or does it indirectly introduce more complexity into stm32h7s78_dk-common.dtsi?

If this addition is welcome, I would be happy to open a PR for this, with specified pins.

[erwango]

@bklaric1, I think that as lnog as the console remains available this should be ok.
Tbh, I would actually opt for FDCAN1, as it could be interesting to be able to use CAN with ethernet. But this is really subjective.

[bklaric1]

@erwango Same. I think that FDCAN1 is the better option here.
I will set this up on Monday and make a PR, with explanation that i2c1 and FDCAN1 should not be enabled at the same time, mentioned in code snippet above.