dt-bindings: net: wireless: brcm4329-fmac: add clock description for AP6275P

Not only AP6275P Wi-Fi device but also all Broadcom wireless devices allow
external low power clock input. In DTS the clock as an optional choice in
the absence of an internal clock.

Reviewed-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Jacobe Zang <jacobe.zang@wesion.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Acked-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://patch.msgid.link/20240910-wireless-mainline-v14-2-9d80fea5326d@wesion.com