scmi: correctly configure MMU for SCMI buffer

Align the MMU area for SCMI shared buffer on section size;
use the ALIGN macro in mmu_set_region_dcache_behaviour call.

Since commit d877f8fd ("arm: provide a function for boards init
code to modify MMU virtual-physical map") the parameter of
mmu_set_region_dcache_behaviour need to be MMU_SECTION_SIZE
aligned.

Fixes: 240720e9 ("firmware: scmi: mailbox/smt agent device")
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Etienne Carriere <etienne.carriere@linaro.org>