ARM: integrator: move VGA base assignment

The global vga_base is used for things like getting an early
console on a PCI-based VGA adapter. Move this assignment into
the probe function for the PCI bridge.

Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>