dt-bindings: serial: Update DT binding docs to support SiFive FU740 SoC

Add new compatible strings to the DT binding documents to support SiFive
FU740-C000.

Signed-off-by: Yash Shah <yash.shah@sifive.com>
Link: https://lore.kernel.org/r/1607403341-57214-5-git-send-email-yash.shah@sifive.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>