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

The Arm Versatile Express system features a bridge device that provides
access to various smaller devices like clocks, reset gates and various
sensors.

Extract the second half of the informal vexpress-sysreg.txt binding and
make it proper DT schema compliant. This makes the old .txt binding
redundant, so remove it.

This describes both the actual parent configuration bridge, as well as
all the possible children devices.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220506140533.3566431-8-andre.przywara@arm.com