ARM: 7245/1: S3C64XX: introduce arch/arm/mach-s3c64xx/common.[ch]

This patch introduces common.[ch] which are used only in the
arch/arm/mach-s3c64xx/ directory. The common.c file merges
the cpu.c, irq.c and irq-eint.c which are used commonly on
S3C64XX SoCs and the common.h file replaces with plat/s3c6400.h
and plat/s3c6410.h files.

Cc: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>