arm64: dts: ls1028a: put SAIs into async mode

The LS1028A SoC has only unidirectional SAIs. Therefore, it doesn't make
sense to have the RX and TX part synchronous. Even worse, the RX part
wont work out of the box because by default it is configured as
synchronous to the TX part. And as said before, the pinmux of the SoC
can only be configured to route either the RX or the TX signals to the
SAI but never both at the same time. Thus configure the asynchronous
mode by default.

Signed-off-by: Michael Walle <michael@walle.cc>
Acked-by: Alison Wang <alison.wang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>