dm: board: complete the initialization of the muxes in initr_dm()

This will probe the multiplexer devices that have a "u-boot,mux-autoprobe"
property. As a consequence they will be put in their idle state.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Pratyush Yadav <p.yadav@ti.com>