STM32G0 SYSCLK can be sourced from PLLRCLK. Given that the HSI runs at
16 MHz and the HSE range is 4-48 MHz, the PLL is the only way to reach
64 MHz. This commit adds `ClockSrc::PLL`.
The PLL sources from either HSI16 or HSE, divides it by `m`, and locks
its VCO to multiple `n`. It then divides the VCO by `r`, `p`, and `q`
to produce up to three associated clock signals:
* PLLRCLK is one of the inputs on the SYSCLK mux. This is the main
reason the user will configure the PLL, so `r` is mandatory and
the output is enabled unconditionally.
* PLLPCLK is available as a clock source for the ADC and I2S
peripherals, so `p` is optional and the output is conditional.
* PLLQCLK exists only on STM32G0B0xx, and exists only to feed the
MCO and MCO2 peripherals, so `q` is optional and the output is
conditional.
When the user specifies `ClockSrc::PLL(PllConfig)`, `rcc::init()`
calls `PllConfig::init()` which initializes the PLL per [RM0454]. It
disables the PLL, waits for it to stop, enables the source
oscillator, configures the PLL, waits for it to lock, and then
enables the appropriate outputs. `rcc::init()` then switches the
clock source to PLLRCLK.
`rcc::init()` is now also resonsible for calculating and setting flash
wait states. SYSCLCK < 24 MHz is fine in the reset state, but 24-48 MHz
requires waiting 1 cycle and 48-64 MHz requires waiting 2 cycles. (This
was likely a blocker for anyone using HSE >= 24 MHz, with or without
the PLL.) Flash accesses are now automatically slowed down as needed
before changing the clock source, and sped up as permitted after
changing the clock source. The number of flash wait states also
determines if flash prefetching will be profitable, so that is now
handled automatically too.
[RM0454]: https://www.st.com/resource/en/reference_manual/rm0454-stm32g0x0-advanced-armbased-32bit-mcus-stmicroelectronics.pdf
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