dev-arm: Fix GITS_BASER initialization/access

dev-arm: Fix GITS_BASER initialization/access

The patch is fixing/improving GITS_BASER registers initialization.

* Not using reserved table types anymore (GITS_BASER.TYPE)
* Using write mask for handling WI bits

Change-Id: Ibe24667fdf22b42b86496167c19fc00bbb0ba191
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20328
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>