drivers: syscon: Add generic syscon API

A syscon device is a device managing a memory region containing a set of
registers that are not cohesive enough to represent as any specific type
of device. We need a driver for that because several other drivers could
use the same region at the same time and we need to io-map the region at
boot for MMU enabled platforms.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>

include/drivers/syscon.h

@@ -0,0 +1,70 @@
+/*
+ * Copyright (c) 2021 Carlo Caione <ccaione@baylibre.com>
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/**
+ * @file
+ * @brief Public SYSCON driver APIs
+ */
+
+#ifndef ZEPHYR_INCLUDE_DRIVERS_SYSCON_H_
+#define ZEPHYR_INCLUDE_DRIVERS_SYSCON_H_
+
+/**
+ * @brief SYSCON Interface
+ * @defgroup syscon_interface SYSCON Interface
+ * @ingroup io_interfaces
+ * @{
+ */
+
+#include <zephyr/types.h>
+#include <device.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @brief Get the syscon base address
+ *
+ * This function returns the syscon base address
+ *
+ * @return 0 on error, the base address on success
+ */
+uintptr_t syscon_get_base(void);
+
+/**
+ * @brief Read from syscon register
+ *
+ * This function reads from a specific register in the syscon area
+ *
+ * @param reg The register offset
+ * @param val The returned value read from the syscon register
+ *
+ * @return 0 on success, negative on error
+ */
+int syscon_read_reg(uint16_t reg, uint32_t *val);
+
+/**
+ * @brief Write to syscon register
+ *
+ * This function writes to a specific register in the syscon area
+ *
+ * @param reg The register offset
+ * @param val The value to be written in the register
+ *
+ * @return 0 on success, negative on error
+ */
+int syscon_write_reg(uint16_t reg, uint32_t val);
+
+/**
+ * @}
+ */
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif	/* ZEPHYR_INCLUDE_DRIVERS_SYSCON_H_ */