dt-bindings: display: bridge: lt8912b: Add power supplies

Add Lontium lt8912b power supplies.

Signed-off-by: Stefan Eichenberger <stefan.eichenberger@toradex.com>
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Robert Foss <rfoss@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20231115121338.22959-3-francesco@dolcini.it