[ARM] S3C64XX: Add IRQ_EINT support

Add the necessary code to support IRQ_EINT(x) on
the S3C64XX series of CPUs.

Note, since there is no GPIO configuration support
in the kernel, the irq set_type method does not
configure the relevant pin to interrupt.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>