wcn36xx: Allow firmware name to be overridden by DT

The WLAN NV firmware blob differs between platforms, and possibly
devices, so add support in the wcn36xx driver for reading the path of
this file from DT in order to allow these files to live in a generic
file system (or linux-firmware).

For some reason the parent (wcnss_ctrl) also needs to upload this blob,
so rather than specifying the same information in both nodes wcn36xx
reads the string from the parent's of_node.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Tested-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Tested-by: Aníbal Limón <anibal.limon@linaro.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210824171225.686683-1-bjorn.andersson@linaro.org