arm: add asm/vga.h to mach-footbridge/dc21285.c

arm: add asm/vga.h to mach-footbridge/dc21285.c

In order to fix this:

  CC      arch/arm/mach-footbridge/dc21285.o
arch/arm/mach-footbridge/dc21285.c: In function 'dc21285_preinit':
arch/arm/mach-footbridge/dc21285.c:299: error: 'vga_base' undeclared (first use in this function)
arch/arm/mach-footbridge/dc21285.c:299: error: (Each undeclared identifier is reported only once
arch/arm/mach-footbridge/dc21285.c:299: error: for each function it appears in.)
make[3]: *** [arch/arm/mach-footbridge/dc21285.o] Error 1

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>