dt-bindings: serial: add a new compatible string for SC9860

SC9860 use the same serial device which SC9836 uses, so added a new
compatible string to support SC9860 as well, also added an example
of how to describe this serial device in DT.

Signed-off-by: Chunyan Zhang <chunyan.zhang@spreadtrum.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>