dev-arm: Fix GICv3 LPIs priority value

dev-arm: Fix GICv3 LPIs priority value

Priority bits in the LPI configuration table entry are only the MSBits
([7:2]) and need to be shifted in order to get the real LPI priority
value.

Change-Id: Id04dd4fa9113a32712c73a7094df498de3c0d2b5
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/+/18594
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>