[ARM] 3551/1: S3C24XX: PM code failes to compile with CONFIG_DCACHE_WRITETHROUGH

Patch from Ben Dooks

If CONFIG_CPU_DCACHE_WRITETHOUGH is set, then the
S3C24XX PM code fails to compile, as there is no
need to flush the D-cache, the flush function
arm920_flush_kern_cache_all() is not compiled.

Fix the code to not use this if the config is set.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>