serial: Move asm-sh/sci.h to linux/serial_sci.h.

This header is needed on other architectures as well (namely h8300),
which currently fails to build without this in place. Rather than
duplicating the port definition completely there, just move this to a
common location instead.

This should get h8300 working again for 2.6.25, in addition to the
changes already pushed by Sato-san in -rc2.

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