driver: clock: Add clock controller support in NPCX series.

Add clock controller support for Nuvoton NPCX series. This CL includes:

1. Add clock controller device tree declarations.
2. Introduce clock-cells in yaml file clock tree to get module's source
clock and turn off/on the its clock
3. Clock controller driver implementation.

Signed-off-by: Mulin Chao <MLChao@nuvoton.com>

include/dt-bindings/clock/npcx_clock.h

@@ -0,0 +1,32 @@
+/*
+ * Copyright (c) 2020 Nuvoton Technology Corporation.
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+#ifndef ZEPHYR_INCLUDE_DT_BINDINGS_CLOCK_NPCX_CLOCK_H_
+#define ZEPHYR_INCLUDE_DT_BINDINGS_CLOCK_NPCX_CLOCK_H_
+
+/* clock bus references */
+#define NPCX_CLOCK_BUS_FREERUN     0
+#define NPCX_CLOCK_BUS_LFCLK       1
+#define NPCX_CLOCK_BUS_OSC         2
+#define NPCX_CLOCK_BUS_FIU         3
+#define NPCX_CLOCK_BUS_CORE        4
+#define NPCX_CLOCK_BUS_APB1        5
+#define NPCX_CLOCK_BUS_APB2        6
+#define NPCX_CLOCK_BUS_APB3        7
+#define NPCX_CLOCK_BUS_APB4        8
+#define NPCX_CLOCK_BUS_AHB6        9
+
+/* clock enable/disable references */
+#define NPCX_PWDWN_CTL1            0
+#define NPCX_PWDWN_CTL2            1
+#define NPCX_PWDWN_CTL3            2
+#define NPCX_PWDWN_CTL4            3
+#define NPCX_PWDWN_CTL5            4
+#define NPCX_PWDWN_CTL6            5
+#define NPCX_PWDWN_CTL7            6
+#define NPCX_PWDWN_CTL8            7
+#define NPCX_PWDWN_CTL_NONE        8
+
+#endif /* ZEPHYR_INCLUDE_DT_BINDINGS_CLOCK_NPCX_CLOCK_H_ */