[ARM] No need to include asm/proc-fns.h into asm/system.h

In the old days when arm26/arm32 was combined into the same
architecture, proc-fns.h provided the xchg implementation for
arm26 CPUs.  Since we no longer combine these two, this include
is no longer required.  Remove it.

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