imx8qxpmek: Update the i.MX 8QXP default build to rev C0

imx8qxpmek: Update the i.MX 8QXP default build to rev C0

The current machine configuration support for 8QXP MEK is for rev B0 silicon.
Support for rev C0 is implemented using a machine override `mx8qxpc0`.

Update the default build to rev C0 silicon. Support for rev B0 is provided
with a new machine feature `soc-revb0`. The machine override is removed.

Signed-off-by: Tom Hochstein <tom.hochstein@nxp.com>