soc: st: stm32mp13: add missing arm_mmu.h include

The samples/subsys/llext/shell_loader test fails when running as a
twister test on the stm32mp135f_dk/stm32mp135fxx platform, with the
following error:

soc/st/stm32/stm32mp13x/soc.c:46:36:
error: array type has incomplete element type 'struct arm_mmu_region'
   46 | static const struct arm_mmu_region mmu_regions[] = {

This commit adds the missing arm_mmu.h include to fix the build issue.

Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@foss.st.com>