pxafb: add transparency field to pxafb_mode_info struct

This allows to select either RGB565 (transparency 0) or RGBT555
(transparency 1) from the mode info

Signed-off-by: Pieter Grimmerink <p.grimmerink@inepro.com>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>