ARM: dts: ls1021a-qds: change fpga to simple-mfd device

The FPGA is not really a bus but more like an MFD device.  Change the
compatible string from "simple-bus" to "simple-mfd".  This also fix a
node name issue with simple-bus schema.

Signed-off-by: Li Yang <leoyang.li@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>