tm32: stm32l15xxx_rcc: configure medium performance voltage range and zero...

tm32: stm32l15xxx_rcc: configure medium performance voltage range and zero wait-state when allowed by SYSCLK setting

Zero wait-state for flash can be configured when:
 Range 1 and SYSCLK <= 16 Mhz
 Range 2 and SYSCLK <= 8 Mhz
 Range 3 and SYSCLK <= 4.2 Mhz

Medium performance voltage range (1.5V) can be configured when SYSCLK is up to 16 Mhz and PLLVCO up to 48 Mhz.