firmware: imx: Allow IMX DSP to be selected as module

IMX DSP is only needed by SOF or any other module that
wants to communicate with the DSP. When SOF is build
as a module IMX DSP is forced to be built inside the
kernel image. This is not optimal, so allow IMX DSP
to be built as a module.

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