serial: mpsc: Remove obsolete MPSC driver

Support for the Marvell MV64x60 line of bridge chips that contained
MPSC controllers has been removed and there are no other components
that have that controller so remove its driver.

Signed-off-by: Mark Greer <mgreer@animalcreek.com>
Link: https://lore.kernel.org/r/20190626160553.28518-1-mgreer@animalcreek.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>