ARM: 6881/1: cputype.h uses __attribute_const__ which requires including kernel.h

Issue manifests as:

In file included from arch/arm/mach-pxa/include/mach/hardware.h:62,
                  from arch/arm/mach-pxa/include/mach/gpio.h:28,
                  from /home/jic23/src/kernel/temp-remove/arch/arm/include/asm/gpio.h:5,
                  from include/linux/gpio.h:7,
                  from drivers/staging/iio/gyro/adis16080_core.c:8:
 /home/jic23/src/kernel/temp-remove/arch/arm/include/asm/cputype.h:57: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'read_cpuid_id'
...

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>