[ARM] S3C: Make IRQ_EINT sleep control common

Move the IRQ_EINT sleep control to be available to all
s3c impelmentations. Since s3c_irqext_wake is not large,
place it in arch/arm/plat-s3c/pm.c as adding it to a new
file would be a waste of compile time.

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