dt-bindings: timeriomem_rng: Add entropy quality property

- reg : base address to sample from

- period : wait time in microseconds to use between samples

Optional properties:

- 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.

N.B. currently 'reg' must be four bytes wide and aligned

Example: