drivers: clock: stm32: factorize clock selection macros

Factorize STM32_CLOCK_*_GET() and STM32_MCO_CFGR_*_GET() macros
into a single series of STM32_DT_CLKSEL_*_GET() macros based on
recently introduced new common macros STM32_DT_CLKSEL_*_SHIFT and
STM32_DT_CLKSEL_*_MASK.

Signed-off-by: Etienne Carriere <etienne.carriere@st.com>