dt-bindings: fu740: prci: add YAML documentation for the FU740 PRCI

Add YAML DT binding documentation for the SiFive FU740 PRCI. The
link of unmatched board as follow, the U740-C000 manual would be present
in the same page later.

    https://www.sifive.com/boards/hifive-unmatched



Passes dt_binding_check.

Signed-off-by: Zong Li <zong.li@sifive.com>
Link: https://lore.kernel.org/r/20201126030043.67390-1-zong.li@sifive.com


Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>