dt-bindings: leds: Add LED_FUNCTION_MOBILE for mobile network

Add LED_FUNCTION_MOBILE for LEDs that indicate status of mobile network
connection. This is useful to distinguish those LEDs from LEDs that
indicates status of wired "wan" connection.

example (on stock fw):

IIJ SA-W2 has "Mobile" LEDs that indicate status (no signal, too low,
low, good) of mobile network connection via dongle connected to USB
port.

- no signal: (none, turned off)
-   too low: green:mobile & red:mobile (amber, blink)
-       low: green:mobile & red:mobile (amber, turned on)
-      good: green:mobile (turned on)

Suggested-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240323074326.1428-2-musashino.open@gmail.com


Signed-off-by: Lee Jones <lee@kernel.org>