arm: imx8mp-phycore: move to OF_UPSTREAM

The PHYCORE_IMX8MP is used by the phyBOARD-Pollux. Migrate board to
OF_UPSTREAM. Linux kernel device tree for the board can be used as is,
corresponding U-Boot device tree files are removed. U-Boot tweaks are
kept unchanged.

Signed-off-by: Yannic Moog <y.moog@phytec.de>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Acked-by: Teresa Remmet <t.remmet@phytec.de>

arch/arm/dts/Makefile

@@ -943,7 +943,6 @@ dtb-$(CONFIG_ARCH_IMX8M) += \
 	imx8mp-dhcom-pdk3-overlay-rev100.dtbo \
 	imx8mp-icore-mx8mp-edimm2.2.dtb \
 	imx8mp-msc-sm2s.dtb \
-	imx8mp-phyboard-pollux-rdk.dtb \
 	imx8mq-pico-pi.dtb \
 	imx8mq-kontron-pitx-imx8m.dtb \
 	imx8mq-librem5-r4.dtb