arch: cache: cp15: Add mmu_set_region_dcache_behaviour() when SYS_DCACHE_OFF is enable

Fix following compilation issue when SYS_DCACHE_OFF is enable:
drivers/misc/scmi_agent.c:128: undefined reference to `mmu_set_region_dcache_behaviour'

when SYS_DCACHE_OFF is enable, mmu_set_region_dcache_behaviour() must be
defined.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>