[microtvm][Zephyr] Add project overlay to overwrite device tree configs

[mehrdadh]

This PR adds a mechanism to overwrite device tree configs in zephyr project. It also adds an overlay for nucleo_l4r5zi board to change processor frequency from 80MHz which is the default to 120MHz.

cc @alanmacd @areusch @gromero @guberti

[areusch]

Thanks @mehrdadh

[guberti]

LGTM - thanks for this change!

[gromero]

@mehrdadh Nice findings, I wonder if you would like to upstream it to Zephyr also so other projects could pick this up?

[erwango]

That is not correct.
You should update the PLL configuration accordingly:

&pll {
	div-m = <2>;
	mul-n = <30>;
	div-p = <7>;
	div-q = <2>;
	div-r = <2>;
	clocks = <&clk_hsi>;
	status = "okay";
};

Note that the impacts of effects on bus configurations have to be tested.

[gromero]

@erwango Thanks a lot. Do you mind to tell how exactly one finds the params (M, N, P, Q, and R) for the PLL? I see them in the clock tree in RM0432 rev. 9 (Arm Reference Manual), Figure 16, but could not find any formula about how to relate them to the 120 MHz freq.

[erwango]

@erwango Thanks a lot. Do you mind to tell how exactly one finds the params (M, N, P, Q, and R) for the PLL? I see them in the clock tree in RM0432 rev. 9 (Arm Reference Manual), Figure 16, but could not find any formula about how to relate them to the 120 MHz freq.

Core freq = Input freq (HSI: 16MHz) / M(2) * N(30) / R(2)

[gromero]

@erwango Thanks for the explanation. I found also the formula in the Reference Manual, below the bit descriptions for the RCC_PLLCFGR, where also there are formulas using the Q and P params. Could you please review #12756 ?

[mehrdadh]

@erwango thanks for catching this and @gromero for the follow up PR!