[ARM] S3C64XX: Add IRQ PM code

Add support for saving the state of the IRQ registers over suspend.

This requires moving the S3C64XX UART registers into <plat/regs-serial.h>
and adding irq-pm.c which saves the state of all the IRQ registers.

The irq-pm.c saves all the IRQ registers, including the IRQ_EINT and
IRQ_EINT_GROUP registers as it was easier than adding three different
files. Also ensuring that all the registers are restored to the same
state as before suspend is considered to be the best thing to do.

Note, we do not suspend the VIC here, this is done by the VIC driver
itself.

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