drivers/clock_control: stm32 common move pllx defines to common header

All these series share the same defines, and while they are not used
by all socs of the common-driver, this is not exptectd to lead to
any conflict.
By moving the defines they can also be used in clock_stm32_ll_common.c

Additionally, the stm32g0 pll_div define was renamed to pllm
in order to match the other series.

Signed-off-by: Thomas Stranger <thomas.stranger@outlook.com>