RISC-V: Add DT documentation for SiFive L2 Cache Controller

Add device tree bindings for SiFive FU540 L2 cache controller driver
Signed-off-by: Yash Shah <yash.shah@sifive.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>

Documentation/devicetree/bindings/riscv/sifive-l2-cache.txt

+SiFive L2 Cache Controller
+--------------------------
+The SiFive Level 2 Cache Controller is used to provide access to fast copies
+of memory for masters in a Core Complex. The Level 2 Cache Controller also
+acts as directory-based coherency manager.
+All the properties in ePAPR/DeviceTree specification applies for this platform
+
+Required Properties:
+--------------------
+- compatible: Should be "sifive,fu540-c000-ccache" and "cache"
+
+- cache-block-size: Specifies the block size in bytes of the cache.
+  Should be 64
+
+- cache-level: Should be set to 2 for a level 2 cache
+
+- cache-sets: Specifies the number of associativity sets of the cache.
+  Should be 1024
+
+- cache-size: Specifies the size in bytes of the cache. Should be 2097152
+
+- cache-unified: Specifies the cache is a unified cache
+
+- interrupts: Must contain 3 entries (DirError, DataError and DataFail signals)
+
+- reg: Physical base address and size of L2 cache controller registers map
+
+Optional Properties:
+--------------------
+- next-level-cache: phandle to the next level cache if present.
+
+- memory-region: reference to the reserved-memory for the L2 Loosely Integrated
+  Memory region. The reserved memory node should be defined as per the bindings
+  in reserved-memory.txt
+
+
+Example:
+
+	cache-controller@2010000 {
+		compatible = "sifive,fu540-c000-ccache", "cache";
+		cache-block-size = <64>;
+		cache-level = <2>;
+		cache-sets = <1024>;
+		cache-size = <2097152>;
+		cache-unified;
+		interrupt-parent = <&plic0>;
+		interrupts = <1 2 3>;
+		reg = <0x0 0x2010000 0x0 0x1000>;
+		next-level-cache = <&L25 &L40 &L36>;
+		memory-region = <&l2_lim>;
+	};