ARM: sa1100: include linux/io.h in hackkit leds code

The sa1100 definition of the io_p2v macro has changed in v3.6, and this one
file stopped working because of that.

Without this patch, building hackkit_defconfig results in:

arch/arm/mach-sa1100/leds-hackkit.c: In function 'hackkit_leds_event':
arch/arm/mach-sa1100/leds-hackkit.c:39:4: error: implicit declaration of function 'IOMEM' [-Werror=implicit-function-declaration]

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Russell King <linux@arm.linux.org.uk>