Some drivers that claim to support mrvl,mmp-uart default to a reg-shift
of two, some don't. Be explicit to be on a safe side.

With that in place, a XScale serial port driver is perfectly capable of
supporting the MMP serial port. Add a compatible string.
Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200320174107.29406-4-lkundrak@v3.skSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>