rockchip: ringneck-px30: disable broken USB gadget
USB gadget simply doesn't work right now on PX30. Tested on PX30 EVB and
PX30 Ringneck with Linux mainline and Rockchip 5.10 Linux as well as
U-Boot.
We don't want to our users to assume that USB gadget is supported on Q7
USB P1 on Ringneck Haikou, so let's remove its support, which also
removes the ums CLI command, fastboot and Android image booting
support.