ARM: S3C: Update CPU register mapping practices.

Currently map-base.h defines the main virtual address mappings made
for all the support S3C SoC series, but does not then define any base
for per-cpu mappings to be made from.

Add S3C_ADDR_CPU() macro to define an virtual address as an offset
from the last of the core mappings.

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