video: xilinxfb: Group bus initialization

Move of_address_to_resource() to xilinxfb_assign()
which simplify driver probing.

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>