ARM: dts: imx: drop redundant 'u-boot,panel-name' property

Panel timing bindings do not allow 'u-boot,panel-name' and there seems
to be no users of it: neither Linux kernel drivers, nor U-boot as of
v2024.07-rc2.  Reported by dtbs_check:

  imx6qp-tx6qp-8037.dtb: display-timings: timing-et0700: 'u-boot,panel-name' does not match any of the regexes: 'pinctrl-[0-9]+'

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