arm/p2m: Cleanup access to the host's p2m

arm/p2m: Cleanup access to the host's p2m

This commit substitutes the direct access of the host's p2m
(&d->arch.p2m) for the macro "p2m_get_hostp2m". This macro simplifies
readability and also the differentiation between the host's p2m and
alternative p2m's, i.e., as part of the altp2m subsystem that will be
submitted in the future.

Signed-off-by: Sergej Proskurin <proskurin@sec.in.tum.de>
Acked-by: Julien Grall <julien.grall@arm.com>