Audio: Up_down_mixer: Convert stubs with arithmetic to generic C

This patch adds more conversions including simple arithmetitic
operations.

Signed-off-by: Seppo Ingalsuo <seppo.ingalsuo@linux.intel.com>

Author: singalsu

src/audio/up_down_mixer/up_down_mixer_generic.c

@@ -5,6 +5,7 @@
 // Author: Bartosz Kokoszko <bartoszx.kokoszko@intel.com>
 //         Seppo Ingalsuo <seppo.ingalsuo@linux.intel.com>
 
+#include <sof/audio/format.h>
 #include <errno.h>
 #include <stddef.h>
 #include <stdint.h>
@@ -274,6 +275,15 @@ void downmix32bit_7_1(struct up_down_mixer_data *cd, const uint8_t * const in_da
 void downmix16bit_stereo(struct up_down_mixer_data *cd, const uint8_t * const in_data,
 			 const uint32_t in_size, uint8_t * const out_data)
 {
+	int32_t sum;
+	const int16_t *in_data16 = (int16_t *)in_data;
+	int16_t *out_data16 = (int16_t *)out_data;
+	int idx;
+
+	for (idx = 0; idx < (in_size / 4); ++idx) {
+		sum = (int32_t)in_data16[2 * idx] + in_data16[2 * idx + 1];
+		out_data16[idx] = sat_int16((sum + 1) >> 1);
+	}
 }
 
 void shiftcopy16bit_mono(struct up_down_mixer_data *cd, const uint8_t * const in_data,
@@ -325,7 +335,15 @@ void downmix16bit_4ch_mono(struct up_down_mixer_data *cd, const uint8_t * const
 void downmix32bit_stereo(struct up_down_mixer_data *cd, const uint8_t * const in_data,
 			 const uint32_t in_size, uint8_t * const out_data)
 {
-	sof_panic(0);
+	int64_t sum;
+	int32_t *in_data32 = (int32_t *)in_data;
+	int32_t *out_data32 = (int32_t *)out_data;
+	int idx;
+
+	for (idx = 0; idx < (in_size / 4); idx++) {
+		sum = (int64_t)in_data32[2 * idx] + in_data32[2 * idx + 1];
+		out_data32[idx] = sat_int32((sum + 1) >> 1);
+	}
 }
 
 void downmix32bit_3_1_mono(struct up_down_mixer_data *cd, const uint8_t * const in_data,