serial: sh-sci: Kill off unused membase kludge.

All users of the platform port data specify a mapbase where the driver
later derives the membase from. Now that UPF flags are taken in to
account for generic ioremapping we can kill off the port-specific membase
clobbering and simply use the generic paths.

This derives from a time when sh64 was not capable of using the generic
ioremap implementation and had employed early bolted DTLB mappings for
port access, which is no longer an issue.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>