gicv3: Define PA masks for the PENDBASR and PROPBASER registers

Define masks for these registers to find the PA of where these are
pointing.

Sponsored by: Netflix
Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D44096

(cherry picked from commit a3f164a0c7af17b6b1903e8f13d1b70072e1ccd0)