firmware: imx: Introduce imx_dsp_setup_channels

Create a separate function that sets up DSP mailbox channels
so that imx_dsp_probe function will be easier to read.

Signed-off-by: Daniel Baluta <daniel.baluta@nxp.com>
Reviewed-by: Paul Olaru <paul.olaru@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>