rockchip: ringneck-px30: enable i2c command

This is a useful tool to check the presence of a device on a specific
i2c bus, so let's enable it.

Cc: Quentin Schulz <foss+uboot@0leil.net>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>