sunxi: display: Mark sunxi_rgb2yuv_coef array as const

sunxi_rgb2yuv_coef is readonly and never modified.

Signed-off-by: Priit Laes <plaes@plaes.org>
Acked-by: Anatolij Gustschin <agust@denx.de>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>