video: mxsfb: Fix framebuffer corruption on mx6sx

Allocate the framebuffer memory as coherent, otherwise the framebuffer
will suffer from artifacts when displaying scrolling text or video.
This can be replicated on i.MX6SX (armv7), which has more complex memory
architecture compared to the i.MX23/28 (armv5).

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>