arm: Add support for KERNEL_TEXT_RDONLY

When using FORCE_PAGES to allocate the kernel memory into pages,
provide an option to mark the the kernel text section as read only.
Since the kernel text pages are always mapped in the kernel, anyone
can write to the page if they have the address.
Enable this option to mark the kernel text pages as read only to
trigger a fault if any code attempts to write to a page that is
part of the kernel text section.

CRs-Fixed: 761629
Change-Id: I27de98e2e82cf6dfe9370695239e1fae26ad0e97
Signed-off-by: Prasad Sodagudi <psodagud@codeaurora.org>