arm64: dts: ls1028a: fix FlexSPI clock

Now that we have a proper driver for the FlexSPI interface use it. This
will fix SCK frequency switching on Layerscape SoCs.

This was tested on the Kontron sl28 board.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>