ARM: DT: msm: Add Qualcomm's PRNG driver binding document

This adds Qualcomm PRNG driver device tree binding documentation
to use as an example in dts trees.

Signed-off-by: Stanimir Varbanov <svarbanov@mm-sol.com>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>