Shared memory based communication of a random seed from one cpu to a
different cpu with external synchronisation.

Required driver node properties:
  - compatible : Should be "avm,shm-rng-seed"
  - memory-region: phandle to a node of reserved memory (see
    Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt)
    used to store the random seed between writing by the sending cpu
    and reading by the receiving cpu.

Optional driver properties:
  - avm,write : Should be set on the writing side (e.g. the first kernel
                started) and not set on the reading side (e.g. the second
                kernel started)

Optional reserved-memory node properties:
  - compatible : Should be "avm,shm-rng-seed-early" if early loading of the
                 memory seed is desired and the driver was compiled into
                 the kernel.

Example for the second kernel reading the data early:
/ {
	...
	reserved-memory {
		shm_rng_seed_mem: shm-rng-seed-mem@12345678 {
			compatible = "avm,shm-rng-seed-early";
			reg = <0x12345678 0x00001000>;
		}
	}
	...
}

Example for the second kernel reading the data late:
/ {
	...
	reserved-memory {
		shm_rng_seed_mem: shm-rng-seed-mem@12345678 {
			reg = <0x12345678 0x00001000>;
		}
	}
	...
	shm-rng-seed@12345678 {
		compatible = "avm,shm-rng-seed";
		memory-region = <&shm_rng_seed_mem>;
	}
	...
}

Example for the first kernel writing the data:
/ {
	...
	reserved-memory {
		shm_rng_seed_mem: shm-rng-seed-mem@12345678 {
			reg = <0x12345678 0x00001000>;
		}
	}
	...
	shm-rng-seed@12345678 {
		compatible = "avm,shm-rng-seed";
		memory-region = <&shm_rng_seed_mem>;
		avm,write;
	}
	...
}