This commit adds the option to use HSE or MSI system clock, and LSE or LSI
RTC clock, on L4 MCUs.
Note that prior to this commit the default clocks on an L4 part were MSI
and LSE. The defaults are now MSI and LSI.
In mpconfigboard.h select the clock source via:
#define MICROPY_HW_RTC_USE_LSE (0) or (1)
#define MICROPY_HW_CLK_USE_HSE (0) or (1)
and the PLLSAI1 N,P,Q,R settings:
#define MICROPY_HW_CLK_PLLSAIN (12)
#define MICROPY_HW_CLK_PLLSAIP (RCC_PLLP_DIV7)
#define MICROPY_HW_CLK_PLLSAIQ (RCC_PLLQ_DIV2)
#define MICROPY_HW_CLK_PLLSAIR (RCC_PLLR_DIV2)
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