arm: mxs: include asm/processor.h for cpu_relax()

I get this build error as of today:

arch/arm/mach-mxs/ocotp.c: In function 'mxs_get_ocotp':
arch/arm/mach-mxs/ocotp.c:54: error: implicit declaration of function 'cpu_relax'
make[2]: *** [arch/arm/mach-mxs/ocotp.o] Error 1

Looks like it has been indirectly included before which broke now.
Include it directly.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Cc: Shawn Guo <shawn.guo@freescale.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>