atmel_lcdfb: support 16bit BGR:565 mode, remove unsupported 15bit modes

Allow framebuffer to be configured in 16bit mode when panel is wired in
(the default) BGR configuration, and don't claim to support 15bit input
modes, which the LCD controller cannot handle.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>