arm/mm: Introduce modify_xen_mappings

arm/mm: Introduce modify_xen_mappings

Which is only used by Livepatch code. The purpose behind
this call is to modify the page table entries flags.

Specifically the .ro and .nx flags. The current mechanism
puts cache attributes in the flags and the .ro and .nx are
locked down and assumed to be .ro=0, nx=1.

Livepatch needs .nx=0 and also .ro to be set to 1.

We introduce a new 'flags' where various bits determine
whether .ro and .nx bits are set or cleared. We can't use
an enum as the function prototype would diverge from x86.

Reviewed-by: Julien Grall <julien.grall@arm.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>