irqchip/gic-v3: Remove inexistant register definition

The GICv3 include file defines GICR_ISACTIVER and GICR_ICACTIVER
in the RD_base page. News flash, they do not exist (probably
a copy/paste brain fart). Just drop them.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>