video: xilinxfb: Add support for little endian accesses

Dynamically detect endianess on IP and use
ioread/iowrite functions instead of powerpc and microblaze
specific out_be32.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>