dts: bindings: stm32: add div-q and div-r on stm32mp13 plls

Depending on the PLL, all DIV-P / DIV-Q and DIV-R are available
on STM32MP13 PLLs.
Adjust valid range in order to be able to set for all 4 PLLs.
Clarify DT properties description.

Signed-off-by: Alain Volmat <alain.volmat@foss.st.com>