serial: sh-sci: remove SCIF code in sci_rxd_in()

Since sci_rxd_in() is used by SCI only, clean up
the header file by killing off code dealing with
SCIF ports and their register definitions.

Also introduce a default sci_rxd_in() function
which can be shared by all SCIF-only processors.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>