Option to use HSE/LSI

[seemefirst]

Added option to use external high speed crystal. To use the HSE/LSI a preprocessor must be defined in stm32l4xx_hal_conf.h as USE_HSE_LSI (1).

[chrismas9]

This is a useful improvement as it allows for accurate timers better than 0.25%. However it is reasonable to want HSE and LSE for accurate timers and RTC. I think selection of HSx and LSx should be separate, like F4. For example PYB has HSE and LSE, but on PYBLITE the 32 kHz crystal is optional . For consistency I suggest you use the macro name MICROPY_HW_CLK_USE_HSE and the existing MICROPY_HW_RTC_USE_LSE.

The PLL values should be defined in mpconfigboard.h, not in system_stm32.c, as some L4 run at 120 MHz, some applications run at less than 80 MHz to save power and some boards use a different crystal frequency. All the PLLSAI1 settings should use macros in mpconfigboard.h as the USB clock divider needs to be set to suit the crystal frequency.

When you make changes to a PR before it is reviewed you can squash the commits and force push.

[chrismas9]

@seemefirst Looks good. Thanks for making the changes.

[dpgeorge]

Thanks for the contribution, merged in 266146a, with follow-up in 26e90a0