dt-bindings: display: sii902x: Add supply bindings

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