[SERIAL] Remove m32r_sio dependency on asm/serial.h

m32r_sio re-uses a custom defined BASE_BAUD from asm/serial.h,
and replaces SERIAL_PORT_DFNS with its own driver private copy.
Since asm/serial.h is supposed to define 8250-based ports using
these symbols, this isn't a sane idea.

Hence, eliminate asm/serial.h from m32r_sio.c.

Acked-by: Hirokazu Takata <takata@linux-m32r.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>