gui: Add BGR565 pixel format support to lvgl

Adds support for the BGR565 pixel format to lvgl. This fixes the lvgl
sample for mimxrt10{50,60,64}_evk boards, which were broken when the
mcux elcdif display driver was modified in commit
9041b0f1 to return the BGR565 pixel
format instead of RGB565.

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>