arm64: 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.

Change-Id: I2a9e105a3340686b4314bb10cc2a6c7bfa19ce8e
Signed-off-by: Rohit Vaswani <rvaswani@codeaurora.org>