dt-bindings: display: sii902x: Add supply bindings

dt-bindings: display: sii902x: Add supply bindings

commit 4c1e054322da99cbfd293a5fddf283f2fdb3e2d0 upstream.

The sii902x chip family requires IO and core voltages to reach the
correct voltage before chip initialization. Add binding for describing
the two supplies.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201020221501.260025-3-mr.nuke.me@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>