mux: autoprobe if "idle-states" present in device tree

Some platforms need the MUX state to be auto initialized at
boot time even if there are no explicit users for the MUX.
In these cases, the MUX device tree has "idle-states" property
which specifies what state the MUX should be initialized to.

So far we were relying on custom u-boot property "u-boot,mux-autoprobe"
to autoprobe such MUXes. This patch causes the MUX to autoprobe
if it has "idle-states" property in device tree.

This should allow us to stop using the custom "u-boot,mux-autoprobe"
property.

Signed-off-by: Roger Quadros <rogerq@kernel.org>