[ARM] S3C64XX: Add EINT group regs and move IRQ_EINT to regs-gpio.h

Add definitions for the EINT group registers and move the EINT IRQ
register definitions out of arch/arm/plat-s3c64xx/irq-eint.c so that
they are available for re-use with PM and the other code.

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