arm64: dts: imx8mp-skov: add basic board as fallback

arm64: dts: imx8mp-skov: add basic board as fallback

All Skov i.MX8MP boards share the same baseboard (modulo revisions)
and are booted with the same bootloader image, which samples some
strapping pins at startup and determines which kernel device tree
to use.

For use as bootloader device tree and as fallback, when no matching
device tree has been found, add a basic variant that doesn't configure
any variant-specific peripherals like displays.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>