drm/tiny: panel-mipi-dbi: Read I/O supply from DT

To support platforms with a separate I/O voltage supply, set the new
io_regulator property along with the regulator property of the DBI
device. Read the I/O supply from a new "io-supply" device tree
property.

Signed-off-by: Otto Pflüger <otto.pflueger@abscue.de>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20221201160245.2093816-3-otto.pflueger@abscue.de