arch: arm: Add macro to set ap[2] access permission bit.

Change to add macro for setting AP[2] bit (access permission bits),
mmu page table descriptor for section mapping.
This is required, as LK uses single level page level translation,
which has format of section descriptor. To make a entry read only,
for all privileged levels, we need to set this bit.

Change-Id: Ifc0dd9ce948a62e7090e8cf7671abe2e6d40fc9c