ARM: 8495/1: ATAGS: move save_atags() to arch/arm/include/asm/setup.h

So it can be used by code outside arch/arm/kernel/. Fix save_atags()
declaration to match its definition while at it.

Signed-off-by: Ivaylo Dimitrov <ivo.g.dimitrov.75@gmail.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>