ARM: includecheck fix: atomic.h

fix the following 'make includecheck' warning:

  arch/arm/include/asm/atomic.h: asm/system.h is included more than once.

Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>