dev-arm: Add several LPI methods in Gicv3Redistributor

dev-arm: Add several LPI methods in Gicv3Redistributor

Refactoring the existing in code in smaller methods will be crucial when
adding the ITS module, which is a client for the redistributor class and
which will require it to take different actions depending on the command
it receives from software.

List of methods:

* read/writeEntryLPI
Reading/Writing a byte from the LPI pending table

* isPendingLPI
Checks if the pINTID LPI is set. Knowing if an LPI is set is needed by
the MOVI command, which is transfering the pending state from one
redistributor to the other only if the LPI is pending.

Change-Id: If14b1c28ff7f2aa20b12dcd822bf6a490cbe0270
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/+/18596
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>