drivers/clock_control: SAM PMC missing soc.h

The source clock_control_sam_pmc.c can not build without the
symbol SOC_ATMEL_SAM_MCK_FREQ_HZ which is contained in soc.h

This bug only shows itself if CONFIG_ARM_MPU is not enabled,
which probably includes soc.h through the <zephyr/arch/cpu.h>
which is not desired behavior.

This commit adds the missing header, making the source build
regardless of CONFIG_ARM_MPU.

Signed-off-by: Bjarki Arge Andreasen <baa@trackunit.com>