ARM: dts: aspeed: Describe random number device

There is a random number generator that updates a register in the SCU
every second. This is compatible with the timeriomem rng driver in the
kernel.

From the timeriomem_rng bindings:

  quality: estimated number of bits of true entropy per 1024 bits read
  from the rng.  Defaults to zero which causes the kernel's default
  quality to be used instead.  Note that the default quality is usually
  zero which disables using this rng to automatically fill the kernel's
  entropy pool.

As to the recommended value for us to use:

 Rick Altherr <raltherr@google.com> wrote:
 > Quality is #bit of entropy per 1000 bits read.  100 is a
 > conservative value that was suggested by those in the know.

Signed-off-by: Joel Stanley <joel@jms.id.au>