The LS1028A has six LPUART controllers. Add the nodes.

This was tested on a custom board.
Signed-off-by: Michael Walle <michael@walle.cc>
Link: https://lore.kernel.org/r/20200307091302.14881-2-michael@walle.ccSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>