dt-bindings: mxsfb: Indentation cleanup

Clean up the ad-hoc indentation in the documentation, no functional change.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Shawn Guo <shawnguo@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>