rockchip: rk3566-anbernic-rgxx3: Rename defconfig to include SoC name

Rename defconfig to include SoC name, use similar pattern as other
RK356x boards: <soc>-<name>.dts -> <name>-<soc>_defconfig

Suggested-by: Kever Yang <kever.yang@rock-chips.com>
Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>