[ARM] pxafb: add accelerator ID for PXA3xx GCU

Add ID 99 for PXA3xx frame buffers and report it in the pxa frame buffer
conditionally, depending on a new flag in struct pxafb_mach_info.

Signed-off-by: Daniel Mack <daniel@caiaq.de>
Cc: linux-fbdev-devel@lists.sourceforge.net
Cc: Dennis Oliver Kropp <dok@directfb.org>
Cc: Sven Neumann <s.neumann@raumfeld.com>
Cc: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>