PM / AVS: rockchip-io: depend on CONFIG_POWER_AVS

The rockchip io-domain driver currently only depends on ARCH_ROCKCHIP
itself. This makes it possible to select the power-domain driver, but
not the POWER_AVS class and results in the iodomain-driver not getting
build in this case.

So add the additional dependency, which also results in the driver
config option now being placed nicely into the AVS submenu.

Fixes: 662a9586 ("PM / AVS: rockchip-io: add driver handling Rockchip io domains")
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>