clock_control: stm32: Add support for MSI PLL MODE

Add support for Low Speed External 32.768 kHz oscillator (LSE ).
Add support for MSI PLL-Mode offering an automatic calibration feature
in combination with the LSE. This allows the MSI to reach an accurate
+/-0,25% clock perfectly suitable for USB full-speed clock.

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>