gpu: ipu-v3: Add functions to set CSI/IC source muxes

Adds two new functions, ipu_set_csi_src_mux() and ipu_set_ic_src_mux(),
that select the inputs to the CSI and IC respectively. Both muxes are
programmed in the IPU_CONF register.

Signed-off-by: Steve Longerbeam <steve_longerbeam@mentor.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>