dt-bindings: Document STM32MP1 Reset Clock Controller (RCC) bindings

The RCC block is responsible of the management of the clock and reset
generation for the complete circuit.
Signed-off-by: Gabriel Fernandez <gabriel.fernandez@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>

Documentation/devicetree/bindings/clock/st,stm32mp1-rcc.txt

+STMicroelectronics STM32 Peripheral Reset Clock Controller
+==========================================================
+
+The RCC IP is both a reset and a clock controller.
+
+RCC makes also power management (resume/supend and wakeup interrupt).
+
+Please also refer to reset.txt for common reset controller binding usage.
+
+Please also refer to clock-bindings.txt for common clock controller
+binding usage.
+
+
+Required properties:
+- compatible: "st,stm32mp1-rcc", "syscon"
+- reg: should be register base and length as documented in the datasheet
+- #clock-cells: 1, device nodes should specify the clock in their
+  "clocks" property, containing a phandle to the clock device node,
+  an index specifying the clock to use.
+- #reset-cells: Shall be 1
+- interrupts: Should contain a general interrupt line and a interrupt line
+  to the wake-up of processor (CSTOP).
+
+Example:
+	rcc: rcc@50000000 {
+		compatible = "st,stm32mp1-rcc", "syscon";
+		reg = <0x50000000 0x1000>;
+		#clock-cells = <1>;
+		#reset-cells = <1>;
+		interrupts = <GIC_SPI 5 IRQ_TYPE_NONE>,
+			     <GIC_SPI 145 IRQ_TYPE_NONE>;
+	};
+
+Specifying clocks
+=================
+
+All available clocks are defined as preprocessor macros in
+dt-bindings/clock/stm32mp1-clks.h header and can be used in device
+tree sources.
+
+Specifying softreset control of devices
+=======================================
+
+Device nodes should specify the reset channel required in their "resets"
+property, containing a phandle to the reset device node and an index specifying
+which channel to use.
+The index is the bit number within the RCC registers bank, starting from RCC
+base address.
+It is calculated as: index = register_offset / 4 * 32 + bit_offset.
+Where bit_offset is the bit offset within the register.
+
+For example on STM32MP1, for LTDC reset:
+ ltdc = APB4_RSTSETR_offset / 4 * 32 + LTDC_bit_offset
+      = 0x180 / 4 * 32 + 0 = 3072
+
+The list of valid indices for STM32MP1 is available in:
+include/dt-bindings/reset-controller/stm32mp1-resets.h
+
+This file implements defines like:
+#define LTDC_R	3072