The Arm Versatile Express system control register block provides GPIO
functionality to some devices and is also used for board identification.

Extract the first half of the informal vexpress-sysreg.txt binding and
make it proper DT schema compliant.
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220506140533.3566431-7-andre.przywara@arm.com