arm64: Fix page permissions for platforms running at EL2

arm64: Fix page permissions for platforms running at EL2

We currently set both and print both PXN and UXN bits when removing
execution for pages. This happens even in the existing per platform
definitions of 'struct mm_region'.

That's not entirely correct though. For stage-1 translations, if a
platform runs on a translation regime with a single privilege level or the
the translation regime supports two privilege levels and we are not
in EL1&0 with HCR_EL2.{NV, NV1} = {1, 1} only BIT54 (XN) is needed
and BIT53(PXN) is reserved 0.

Currently we support Non-Secure EL2, Non-secure EL2&0 and Non-secure
EL1&0.

We already have get_effective_el() which returns 1 if we are
- Running in EL1 so we assume an EL1 translation regime but without
  checking HCR_EL2.{NV, NV1} != {1,1}
- Running in EL2 with HCR_EL2.E2H = 1

The only problem with the above is that if we are in EL1&0 and
HCR_EL2.{NV1, NV} == {1, 1}, then
- Bit[54] holds the PXN instead of the UXN
- The Effective value of UXN is 0
- Bit[53] is RES0

So let's re-use that function and set PXN only when we are in
and EL[2|1]&0 translation regime.

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>