drivers/clock_control: stm32 common allow pll also when it is not sysclk

The SOC specific implementations of the clock_stm32_ll_common driver
included the PLL specific functions only when PLL was selected as sysclock.

This commit changes the condition from "STM32_SYSCLK_SRC_PLL"
to "defined(STM32_PLL_ENABLED)".
As a result the pll could also be used as peripheral clock source
in case it is not the sysclock.

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