drivers: syscon: Add support for multiple regions

1. Add support for multiple syscon entries (as a side effect, this also
   fixed syscon.c implementations which weren't being linked to their
   syscon.h counterparts).
2. Add support for different width registers in syscon.
3. Add tests for syscon

Signed-off-by: Yuval Peress <peress@chromium.org>

drivers/syscon/syscon_common.h

@@ -0,0 +1,41 @@
+/*
+ * Copyright 2021 Google LLC.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#ifndef DRIVERS_SYSCON_SYSCON_COMMON_H_
+#define DRIVERS_SYSCON_SYSCON_COMMON_H_
+
+#include <sys/util.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @brief Align and check register address
+ *
+ * @param reg Pointer to the register address in question.
+ * @param reg_size The size of the syscon register region.
+ * @param reg_width The width of a single register (in bytes).
+ * @return 0 if the register read is valid.
+ * @return -EINVAL is the read is invalid.
+ */
+static inline int syscon_sanitize_reg(uint16_t *reg, size_t reg_size, uint8_t reg_width)
+{
+	/* Avoid unaligned readings */
+	*reg = ROUND_DOWN(*reg, reg_width);
+
+	/* Check for out-of-bounds readings */
+	if (*reg >= reg_size) {
+		return -EINVAL;
+	}
+
+	return 0;
+}
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* DRIVERS_SYSCON_SYSCON_COMMON_H_ */