gicv3: Split out finding the page size

When adding indirect (2 level) tabled we will need to know the page
size to calculate the size of the level 1 table. To allow for this find
the page size before entering the loop to calculate the final register
value.

Reviewed by: gallatin, imp
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D41551

(cherry picked from commit 3fc4f7c88085dc94c88348839074b8db6f19fae1)