ARM: s5p64x0: fix building with only one soc type

The s5p64x0 platform supports two distinct SoCs, s5p6440 and s5p6450,
and in the normal configuration, both are enabled. However if we build
a kernel that only enables one of the two, the #ifdef logic in common.c
breaks down, as some of the functions declared in the header are defined
to NULL using the preprocessor but then defined anyway.

This patch cleans up the mess and ensures that each function has either
exactly one C declaration and one matching C definition, or we have
a NULL defined function pointer but no C definition.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Kukjin Kim <kgene.kim@samsung.com>
Cc: Tomasz Figa <tomasz.figa@gmail.com>
Cc: Ben Dooks <ben-linux@fluff.org>