atmel_lcdfb: use spare bits in 32bpp mode as alpha channel

Set var->transp.offset and var->transp.length in 32bpp mode to indicate
that the 8 otherwise unused bits can be used for transparency.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@rfo.atmel.com>
Cc: "Antonino A. Daplas" <adaplas@pol.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>