[ARM] Setting read-only section MMU entries as read only.

The linker script sections have been reorganized and split to set different
places for read-only and read-write sections, and the MMU L2 entries are now
set as priviledged read-write access or priviledged read-only access.
".init.data" have been renamed ".initdata" to avoid initialization data
to fall in the linker script ".init" section.
Uninitialized data have been set into the '.bss' sections.