malta: enable PIIX4 SERIRQ

Whilst U-boot does not require this itself, Linux currently relies upon
it having been muxed and enabled by the bootloader. Thus in order to
preserve compatibility with current kernels before a fix is merged in
Linux we will enable the SERIRQ interrupt and mux it to its pin.

Without doing this current kernels will never receive serial port
interrupts and the end result is typically that userland appears to
hang.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>