ARM: set vga memory base at run-time

Convert the incorrectly named PCIMEM_BASE to a variable called vga_base.
This removes the dependency on mach/hardware.h.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>