leds: Add LED class driver for regulator driven LEDs.

This driver provides an interface for controlling LEDs (or vibrators)
connected to PMICs for which there is a regulator framework driver.

This driver can be used, for instance, to control vibrator on all Motorola EZX
phones using the pcap-regulator driver services.

Signed-off-by: Antonio Ospite <ospite@studenti.unina.it>
Reviewed-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>