ASoC: SOF: imx8: Fix randbuild error

when do randconfig like this:
CONFIG_SND_SOC_SOF_IMX8_SUPPORT=y
CONFIG_SND_SOC_SOF_IMX8=y
CONFIG_SND_SOC_SOF_OF=y
CONFIG_IMX_DSP=m
CONFIG_IMX_SCU=y

there is a link error:

sound/soc/sof/imx/imx8.o: In function 'imx8_send_msg':
imx8.c:(.text+0x380): undefined reference to 'imx_dsp_ring_doorbell'

Select IMX_DSP in SND_SOC_SOF_IMX8_SUPPORT to fix this

Fixes: f9ad7546 ("ASoC: SOF: imx: fix reverse CONFIG_SND_SOC_SOF_OF dependency")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Daniel Baluta <daniel.baluta@nxp.com>
Link: https://lore.kernel.org/r/20200409071832.2039-2-daniel.baluta@oss.nxp.comSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/sof/imx/Kconfig

@@ -14,7 +14,7 @@ if SND_SOC_SOF_IMX_TOPLEVEL
 config SND_SOC_SOF_IMX8_SUPPORT
 	bool "SOF support for i.MX8"
 	depends on IMX_SCU
-	depends on IMX_DSP
+	select IMX_DSP
 	help
 	  This adds support for Sound Open Firmware for NXP i.MX8 platforms
 	  Say Y if you have such a device.