firmware: imx: Remove call to devm_of_platform_populate

IMX DSP device is created by SOF layer. The current call to
devm_of_platform_populate is not needed and it doesn't produce
any effects.

Fixes: ffbf23d5 ("firmware: imx: Add DSP IPC protocol interface)
Signed-off-by: Daniel Baluta <daniel.baluta@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>