Add basic ADC support

Planning on abstracting this behind hardware setup APIs, but this works
for a simple demo until then

core/hardware/adc.c

@@ -0,0 +1,75 @@
+#include <inttypes.h>
+#include <stddef.h>
+#include <stdint.h>
+
+#include <zephyr/device.h>
+#include <zephyr/devicetree.h>
+#include <zephyr/drivers/adc.h>
+#include <zephyr/kernel.h>
+
+/** Setup code referenced from Zephyr ADC example **/
+#if !DT_NODE_EXISTS(DT_PATH(zephyr_user)) || \
+	!DT_NODE_HAS_PROP(DT_PATH(zephyr_user), io_channels)
+#error "No suitable devicetree overlay specified"
+#endif
+
+#define DT_SPEC_AND_COMMA(node_id, prop, idx) \
+    ADC_DT_SPEC_GET_BY_IDX(node_id, idx),
+
+static const struct adc_dt_spec adc_channels[] = {
+    DT_FOREACH_PROP_ELEM(DT_PATH(zephyr_user), io_channels, DT_SPEC_AND_COMMA)
+};
+
+static int setup = 0;
+
+int
+adc_init()
+{
+    for (int i = 0; i < ARRAY_SIZE(adc_channels); i++) {
+        int err;
+        const struct adc_dt_spec *chan = &adc_channels[i];
+
+        if (!adc_is_ready_dt(chan)) {
+            return (-1);
+        }
+
+        err = adc_channel_setup_dt(chan);
+        if (err) {
+            return (err);
+        }
+    }
+
+    setup = 1;
+    return (0);
+}
+
+int
+adc_get_mv(int channel)
+{
+    uint16_t val_raw;
+    int32_t val_mv;
+    struct adc_sequence seq = {
+        .buffer = &val_raw,
+        .buffer_size = sizeof(val_raw)
+    };
+    const struct adc_dt_spec *chan;
+
+    if ((channel >= ARRAY_SIZE(adc_channels)) ||
+        (!setup)) {
+        return (INT32_MIN);
+    }
+
+    chan = &adc_channels[channel];
+    adc_sequence_init_dt(chan, &seq);
+
+    if (adc_read_dt(chan, &seq) < 0) {
+        return (INT32_MIN);
+    }
+
+    val_mv = (int32_t)val_raw;
+    if (adc_raw_to_millivolts_dt(chan, &val_mv)) {
+        return (INT32_MIN);
+    }
+
+    return (val_mv);
+}

core/hardware/adc.h

@@ -0,0 +1,11 @@
+#ifndef _ADC_H_
+#define _ADC_H_
+
+#include "includes/hardware/adc.h"
+
+/**
+ * @brief Initialize ADC hardware
+ */
+int adc_init(void);
+
+#endif /* _ADC_H_ */