tty: serial8250: allow platforms to override irq handler

Some ports (e.g. Synopsys DesignWare 8250) have special requirements for
handling the interrupts.  Allow these platforms to specify their own
interrupt handler that will override the default.
serial8250_handle_irq() is provided so that platforms can extend the IRQ
handler rather than completely replacing it.

Signed-off-by: Jamie Iles <jamie@jamieiles.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>