dt-bindings: serial: lpuart: add ls1028a compatibility

Add the LS1028A SoC compatibility string to the lpuart devicetree
bindings documentation.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200307091302.14881-1-michael@walle.cc


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