ASoC: SOF: imx8: Fix COMPILE_TEST error

When do compile test, if SND_SOC_SOF_OF is not set, we get:

sound/soc/sof/imx/imx8.o: In function `imx8_dsp_handle_request':
imx8.c:(.text+0xb0): undefined reference to `snd_sof_ipc_msgs_rx'
sound/soc/sof/imx/imx8.o: In function `imx8_ipc_msg_data':
imx8.c:(.text+0xf4): undefined reference to `sof_mailbox_read'
sound/soc/sof/imx/imx8.o: In function `imx8_dsp_handle_reply':
imx8.c:(.text+0x160): undefined reference to `sof_mailbox_read'

Make SND_SOC_SOF_IMX_TOPLEVEL always depends on SND_SOC_SOF_OF

Reported-by: Hulk Robot <hulkci@huawei.com>
Fixes: 202acc56

 ("ASoC: SOF: imx: Add i.MX8 HW support")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Daniel Baluta <daniel.baluta@nxp.com>
Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20190905064400.24800-1-yuehaibing@huawei.com
Signed-off-by: Mark Brown <broonie@kernel.org>