[ARM] pxafb: add support for FBIOPAN_DISPLAY by dma braching

dma branching is enabled by extending the current setup_frame_dma()
function to allow a 2nd set of frame/palette dma descriptors to be
used.

As a result, pxafb_dma_buff.dma_desc[], pxafb_dma_buff.pal_desc[]
and pxafb_info.fdadr[] are doubled.

This allows maximum re-use of the current dma setup code, although
the pxafb_info.fdadr[xx] for FBRx register values looks a bit odd.

Signed-off-by: Eric Miao <eric.miao@marvell.com>
Signed-off-by: Eric Miao <ycmiao@ycmiao-hp520.(none)>