ARM: dts: imx: correct choice of panel native mode

Bindings and Linux driver expect native-mode to be a phandle to one of
the timings node, not a boolean property.  Correct the DTS to fix
dtbs_check warnings like:

  imx53-m53evk.dtb: display-timings: timing-800x480p60: 'native-mode' does not match any of the regexes: 'pinctrl-[0-9]+'

This should not have actual effect for Linux kernel (no real bug
affecting choice of native-mode), because the first timing node is
chosen in absence of proper native-mode property.

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