ARM: mach-orion5x: add missing header file <linux/vga.h>

This patch fixed following building error:
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arch/arm/mach-orion5x/pci.c: In function 'orion5x_pci_sys_setup':
arch/arm/mach-orion5x/pci.c:563:2: error: 'vga_base' undeclared (first use in this function)
arch/arm/mach-orion5x/pci.c:563:2: note: each undeclared identifier is reported only once for each function it appears in
make[1]: *** [arch/arm/mach-orion5x/pci.o] Error 1
make[1]: *** Waiting for unfinished jobs....
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Signed-off-by: Bryan Wu <bryan.wu@canonical.com>
Acked-by: Rob Herring <rob.herring@calxeda.com>