ARM: dts: imx7d-nitrogen7: Fix the description of the Wifi clock

According to bindings/regulator/fixed-regulator.txt the 'clocks' and
'clock-names' properties are not valid ones.

In order to turn on the Wifi clock the correct location for describing
the CLKO2 clock is via a mmc-pwrseq handle, so do it accordingly.

Fixes: 56354959

 ("ARM: dts: imx: add Boundary Devices Nitrogen7 board")
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Acked-by: Troy Kisky <troy.kisky@boundarydevices.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>