drm/bridge: lt8912b: Add power supplies

Add supplies to the driver that can be used to turn the Lontium lt8912b
on and off. It can have up to 7 independent supplies, we add them all
and enable/disable them with bulk_enable/disable.

Signed-off-by: Stefan Eichenberger <stefan.eichenberger@toradex.com>
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Reviewed-by: Robert Foss <rfoss@kernel.org>
Signed-off-by: Robert Foss <rfoss@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20231115121338.22959-4-francesco@dolcini.it