ARM: mach-footbridge: add missing header file <video/vga.h>

ARM: mach-footbridge: add missing header file <video/vga.h>

This patch fixes following building error:
--
arch/arm/mach-footbridge/dc21285.c: In function 'dc21285_preinit':
arch/arm/mach-footbridge/dc21285.c:299:2: error: 'vga_base' undeclared (first use in this function)
arch/arm/mach-footbridge/dc21285.c:299:2: note: each undeclared identifier is reported only once for each function it appears in
make[1]: *** [arch/arm/mach-footbridge/dc21285.o] Error 1
--

Signed-off-by: Bryan Wu <bryan.wu@canonical.com>
Acked-by: Rob Herring <rob.herring@calxeda.com>