Per display-timings.yaml, the 'timing' pattern should be used to
describe the display timings.

Change it accordingly to fix the following dt-schema warning:

imx27-apf27dev.dtb: display-timings: '800x480' does not match any of the regexes: '^timing', 'pinctrl-[0-9]+'
	from schema $id: http://devicetree.org/schemas/display/panel/display-timings.yaml#Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>