phycore-rk3288: Migrate to OF_UPSTREAM

The phycore-rk3288 can be migrated to OF_UPSTREAM.
Drop redundant device tree files, update MAINTAINERS and
defconfig's DEFAULT_DEVICE_TREE for rockchip vendor prefix accordingly.
Move device tree nodes required for u-boot to rk3288-phycore-rdk-u-boot.dtsi
such as chosen, i2c_eeprom_id and it's alias.

Signed-off-by: Wadim Egorov <w.egorov@phytec.de>