ARM: dts: imx7s: Fix lcdif compatible

[ Upstream commit 5f55da4cc37051cda600ea870ce8cf29f1297715 ]

imx7d-lcdif is compatible to imx6sx-lcdif. MXSFB_V6 supports overlay
by using LCDC_AS_CTRL register. This registers used by overlay plane:
* LCDC_AS_CTRL
* LCDC_AS_BUF
* LCDC_AS_NEXT_BUF
are listed in i.MX7D RM as well.

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>