serial: split serial.c

serial: split serial.c

Split serial.c into serial.c, serial.h and serial-isa.c.  While being at
creating a serial.h header file move the serial prototypes from pc.h to
the new serial.h.  The latter leads to s/pc.h/serial.h/ in tons of
boards which just want the serial bits from pc.h

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>