net: dsa: qca8k: add LEDs blink_set() support

Add LEDs blink_set() support to qca8k Switch Family.
These LEDs support hw accellerated blinking at a fixed rate
of 4Hz.

Reject any other value since not supported by the LEDs switch.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Pavel Machek <pavel@ucw.cz>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>