arm64: dts: ls1028a: fix FlexSPI clock input

On the LS1028A the FlexSPI clock is connected to the first HWA output,
see Figure 7 "Clock subsystem block diagram".

Fixes: c77fae5b

 ("arm64: dts: ls1028a: Add FlexSPI support")
Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>