ARM: dts: imx7d-pico: Describe the Wifi clock

The Wifi chip should be clocked by a 32kHz clock coming from i.MX7D
CLKO2 output pin, so describe the pinmux and clock hierarchy in the
device tree to allow the Wifi chip to be properly clocked.

Managed to successfully test Wifi with such change. Used the standard
nvram.txt file provided by TechNexion, which selects an external 32kHz
clock for the Wifi chip by default.

Fixes: 99a52450 ("ARM: dts: imx7d-pico: Add Wifi support")
Suggested-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Tested-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>