arm64: Don't set executable permissions for pages

For memory security requirements, we should never have
executable and writable pages at the same time. Change the
page permissions from PAGE_KERNEL_EXEC to just PAGE_KERNEL.

Change-Id: I7af470908bd02eee948a9d8934c90e9e44f06570
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>