video: mb862xxfb: relocate register space to get contiguous vram

By default the GDC registers are located in the middle of the 64MiB
area for video RAM and registers. When 32MiB VRAM or more is used,
relocate the register space to the top of the 64MiB space so that
we get the contiguous VRAM for GDC frame buffer layers, drawing
frames, capture and cursor buffers.

Signed-off-by: Anatolij Gustschin <agust@denx.de>