omapfb: fix broken build on OMAP1

Fix the following build regression in 3.9-rc1 by including
<mach/hardware.h>:

drivers/video/omap/omapfb_main.c: In function 'set_fb_var':
drivers/video/omap/omapfb_main.c:505:3: error: implicit declaration of function 'cpu_is_omap15xx' [-Werror=implicit-function-declaration]

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>