ARM: 8403/1: kbuild: don't use generic mcs_spinlock.h header

We provide our own implementation of asm/mcs_spinlock.h, so there's no
need to ask for the (empty) generic version.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>