dt-bindings: arm: convert vexpress-sysregs to DT schema

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