ARM: dts: iwg20d-q7-common: Fix touch controller probe failure

As per the iWave RZ/G1M schematic, the signal LVDS_PPEN controls the
supply voltage for the touch panel, LVDS receiver and RGB LCD panel. Add
a regulator for these device nodes and remove the powerdown-gpios
property from the lvds-receiver node as it results in a touch controller
driver probe failure.

Fixes: 6f89dd9e

 ("ARM: dts: iwg20d-q7-common: Add LCD support")
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://lore.kernel.org/r/20200924080535.3641-1-biju.das.jz@bp.renesas.com
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>