ARM: dts: imx: align panel timings node name with dtschema

DT schema expects panel timings node to follow certain pattern,
dtbs_check warnings:

  imx6dl-gw54xx.dtb: display-timings: 'hsd100pxn1' does not match any of the regexes: '^timing', 'pinctrl-[0-9]+'

Linux drivers do not care about node name, so this should not have
effect on Linux.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>