xen/arm: mm: Rename xenheap_* variable to directmap_*

xen/arm: mm: Rename xenheap_* variable to directmap_*

With the static heap setup, keep using xenheap_* in the function
setup_xenheap_mappings() will make the code confusing to read,
because we always need to map the full RAM on Arm64. Therefore,
renaming all "xenheap_*" variables to "directmap_*" to make clear
the area is used to access the RAM easily.

On Arm32, only the xenheap is direct mapped today. So the renaming
to "directmap_*" would be still valid for Arm32.

As the xenheap_* is renamed to directmap_*, rename the function
setup_xenheap_mappings() to setup_directmap_mappings() to reflect
the variable renaming, also change the code comment and printed
error message in the function accordingly.

No functional change is intended.

Signed-off-by: Henry Wang <Henry.Wang@arm.com>
Acked-by: Julien Grall <jgrall@amazon.com>