config: enable generic S/PDIF codec drivers (#2104)

These drivers can be used as dummy ADC/DAC drivers for
attaching general codecs that don't need to be configured.

This option will build 2 additional drivers, spdif_receiver
and spdif_transmitter.

Since these drivers have DT bindings they are handy for quick
testing of I2S peripherals with simple-audio-card.

eg:

fragment@0 {
    target-path = "/";
    __overlay__ {
        dummy_receiver: spdif-receiver {
            #address-cells = <0>;
            #size-cells = <0>;
            #sound-dai-cells = <0>;
            compatible = "linux,spdif-dir";
            status = "okay";
        };
    };
};

Signed-off-by: Matthias Reichl <hias@horus.com>

arch/arm/configs/bcm2709_defconfig

@@ -898,6 +898,7 @@ CONFIG_SND_PISOUND=m
 CONFIG_SND_SOC_ADAU1701=m
 CONFIG_SND_SOC_ADAU7002=m
 CONFIG_SND_SOC_AK4554=m
+CONFIG_SND_SOC_SPDIF=m
 CONFIG_SND_SOC_WM8804_I2C=m
 CONFIG_SND_SIMPLE_CARD=m
 CONFIG_HIDRAW=y

arch/arm/configs/bcmrpi_defconfig

@@ -891,6 +891,7 @@ CONFIG_SND_PISOUND=m
 CONFIG_SND_SOC_ADAU1701=m
 CONFIG_SND_SOC_ADAU7002=m
 CONFIG_SND_SOC_AK4554=m
+CONFIG_SND_SOC_SPDIF=m
 CONFIG_SND_SOC_WM8804_I2C=m
 CONFIG_SND_SIMPLE_CARD=m
 CONFIG_HIDRAW=y