realtek: add base register field LED support

realtek: add base register field LED support

Realtek LED hardware peripherals share some common features. They are
typically controlled through a register field, a few bits in size. These
allow the LED to be turned off or on, or toggled at one of a number of
intervals.

Support for register field LEDs is added in a new directory, so Realtek
LEDs drivers can be grouped together.

Signed-off-by: Sander Vanheule <sander@svanheule.net>