arm: mm: Compare phys address instead of virt

Doing __va(movable_reserved_start) can overflow if
movable_reserved_start is a very high address. This
will cause the comparison to evaluate incorrectly.

This change instead compares the physical addresses, which
cannot overflow/underflow.

Change-Id: I6c82df16b77a905617aa6f59c2eeaf7acb36c76d
Signed-off-by: Jack Cheung <jackc@codeaurora.org>