video: mxc_ipuv3_fb: stash frame buffer pointer in global data.

This patch updates the i.MX video driver to store the
frame-buffer address in the fb_base field of the global
data structure *gd.

By doing this, you can find the frame buffer address
using the 'bdinfo' command:

	U-Boot > bdinfo
	arch_number = 0x00000EB9
	...
	FB base     = 0x4F35F1C0

This is very useful when debugging display connections.

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>
Acked-by: Otavio Salvador <otavio@ossystems.com.br>