dt-bindings: vexpress-sysreg: Add deprecated GPIO provider properties

While the current binding expects a child GPIO node, the old style binding
with GPIO provider properties in the parent vexpress-sysreg node is
still in use. Add the GPIO provider properties as deprecated.

Cc: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220613171809.3928949-1-robh@kernel.org