dt-bindings: leds: Add LED_FUNCTION_SPEED_* for link speed on LAN/WAN

dt-bindings: leds: Add LED_FUNCTION_SPEED_* for link speed on LAN/WAN

Add LED_FUNCTION_SPEED_LAN and LED_FUNCTION_SPEED_WAN for LEDs that
indicate link speed of ethernet ports on LAN/WAN. This is useful to
distinguish those LEDs from LEDs that indicate link status (up/down).

example:

Fortinet FortiGate 30E/50E have LEDs that indicate link speed on each
of the ethernet ports in addition to LEDs that indicate link status
(up/down).

- 1000 Mbps: green:speed-(lan|wan)-N
-  100 Mbps: amber:speed-(lan|wan)-N
-   10 Mbps: (none, turned off)

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20240323074326.1428-3-musashino.open@gmail.com
Signed-off-by: Lee Jones <lee@kernel.org>