board: rockchip: add Powkiddy X55

The Powkiddy X55 is a Rockchip RK3566 based handheld gaming device.
UART, ADC, eMMC, and SDMMC are tested to work in U-Boot and this
successfully boots mainline Linux.

Kernel commit:
e99adc97e21a ("arm64: dts: rockchip: Add Powkiddy X55")

Signed-off-by: Chris Morgan <macromorgan@hotmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>