stm32f4: SPI clock rates

[dhalbert]

@hierophect Could you check the baudrate calculation in busio.SPI? I was using an SPI clock to try to test the difference between the "bad" and "good" STM32 Feathers I have, and I got odd SPI clock rates on both:

specified	as measured on Saelae
160 MHz	20.83 MHz
40 MHz	10.42 MHz
16 MHz	5.208 MHz
15 MHz	5.208 MHz
8 MHz	2.632 MHz
1 MHz	328 kHz
100 kHz	164 kHz

[hierophect]

@dhalbert Looks like everything is half of what it should be. I'll check it out. In terms of granularity, though, we aren't going to do much better - STM32 has a pretty limited divisor set for SPI. The actual values we're sorting from are:

bus clock
(168MHz / 2) => (PCLK2 = 84000000)

SPI values:
/2 42_000_000
/4 21_000_000
/8 10_500_000
/16 5_250_000
/32 2_625_000
/64 1_312_500
/128 656_250
/256 328_125

[hierophect]

I thought all the SPI clocks were on the PCLK2 bus clock. They are not. Never trust what you read online, folks. Note that SPI2 and SPI3 will have half the maximum speed, since PCLK1 has a max speed of 42MHz.