tty: serial: 8250: Define earlycon for mrvl,mmp-uart

mrvl,pxa-uart already supports earlycon and both compatible strings use
the same driver, so there's no reason for mmp-uart to not have earlycon
as well.

Signed-off-by: Duje Mihanović <duje.mihanovic@skole.hr>
Link: https://lore.kernel.org/r/20230721210042.21535-2-duje.mihanovic@skole.hr
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>