xen/arm: mm: Rework MAIR* definitions to handle 32-bit compilation environment

xen/arm: mm: Rework MAIR* definitions to handle 32-bit compilation environment

Commit a0543df403 "xen/arm: page: Clean-up the definition of MAIRVAL"
combined the definition of MAIR0VAL and MAIR1VAL in MAIRVAL. Sadly, when
building in 32-bit environment, the assembler is unable to compute
64-bit constant and will ignore the 32-bit most-significants bits. This
will result of MAIR1 set 0.

Rather than fully reverting the offending commit, the code is reworked
to still avoid hardcoded values but split the definition in 2.

Lastly, a comment is added to avoid trying to blindly combine the both
definition again in the future.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>