leds: add APIs for LEDs hw control

Add an option to permit LED driver to declare support for a specific
trigger to use hw control and setup the LED to blink based on specific
provided modes.

Add APIs for LEDs hw control. These functions will be used to activate
hardware control where a LED will use the provided flags, from an
unique defined supported trigger, to setup the LED to be driven by
hardware.

Add hw_control_is_supported() to ask the LED driver if the requested
mode by the trigger are supported and the LED can be setup to follow
the requested modes.

Deactivate hardware blink control by setting brightness to LED_OFF via
the brightness_set() callback.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>