video: mb862xx: add option CONFIG_VIDEO_MB862xx_ACCEL for 32bpp mode

The new IPEK01 board can use the 32 bpp mode for the Lime graphics
controller. For this mode, video accelaration does not work. This patch
makes the accelaration configurable via CONFIG_VIDEO_MB862xx_ACCEL,
which is enabled for the lwmon5 and the socrates board for backward
compatibility.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Wolfgang Grandegger <wg@denx.de>