dt-bindings: display: panel: mipi-dbi-spi: Add io-supply

Add documentation for the new io-supply property, which specifies the
regulator for the I/O voltage supply on platforms where the panel
panel power and I/O supplies are separate.

Signed-off-by: Otto Pflüger <otto.pflueger@abscue.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20221201160245.2093816-4-otto.pflueger@abscue.de