xen/arm: Remove the variable dom0_11_mapping and open-code the value

xen/arm: Remove the variable dom0_11_mapping and open-code the value

Dom0 (aka hardware domain on Arm) is always direct mapped. Rather than
using a global variable to store a const, directly open-code it or
replace the use with is_domain_direct_mapped(...) macros.

This will also help a follow-up patch to move all domain_build.c in
init.

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