dt-bindings: firmware: Add bindings for Tegra210 BPMP

The BPMP is a specific processor in Tegra210 chip, which is designed
for boot process handling, assisting in entering deep low power states
(suspend to ram), and offloading DRAM memory clock scaling on some
platforms.
Signed-off-by: Timo Alho <talho@nvidia.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>

Documentation/devicetree/bindings/firmware/nvidia,tegra210-bpmp.txt

+NVIDIA Tegra210 Boot and Power Management Processor (BPMP)
+
+The Boot and Power Management Processor (BPMP) is a co-processor found
+in Tegra210 SoC. It is designed to handle the early stages of the boot
+process as well as to assisting in entering deep low power state
+(suspend to ram), and also offloading DRAM memory clock scaling on
+some platforms. The binding document defines the resources that would
+be used by the BPMP T210 firmware driver, which can create the
+interprocessor communication (IPC) between the CPU and BPMP.
+
+Required properties:
+- compatible
+    Array of strings
+    One of:
+    - "nvidia,tegra210-bpmp"
+- reg: physical base address and length for HW synchornization primitives
+       1) base address and length to Tegra 'atomics' hardware
+       2) base address and length to Tegra 'semaphore' hardware
+- interrupts: specifies the interrupt number for receiving messages ("rx")
+              and for triggering messages ("tx")
+
+Optional properties:
+- #clock-cells : Should be 1 for platforms where DRAM clock control is
+                 offloaded to bpmp.
+
+Example:
+
+bpmp@70016000 {
+	compatible = "nvidia,tegra210-bpmp";
+	reg = <0x0 0x70016000 0x0 0x2000
+	       0x0 0x60001000 0x0 0x1000>;
+	interrupts = <GIC_SPI 6 IRQ_TYPE_EDGE_RISING>,
+		     <GIC_SPI 4 IRQ_TYPE_EDGE_RISING>;
+	interrupt-names = "tx", "rx";
+};