arm64: dts: imx8mp pollux: add expansion board overlay

arm64: dts: imx8mp pollux: add expansion board overlay

An expansion board (PEB-AV-10) may be connected to the
imx8mp-phyboard-pollux. Its main purpose is to provide multimedia
interfaces, featuring a 3.5mm headphone jack, a USB-A port and LVDS as
well as backlight connectors.
Introduce the expansion board as dtsi, as it may be used standalone as
an expansion board, as well as in combination with display panels. These
display panels will include the dtsi.

Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Yannic Moog <y.moog@phytec.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>