leds: add a driver for syscon-based LEDs

This makes it possible to create a set of LEDs from a syscon
MFD instance, which is lean mean and clean on the ARM
reference designs and can replace the Versatile LEDs driver
in the long run, as well as other custom syscon LEDs drivers.

Cc: Bryan Wu <cooloney@gmail.com>
Cc: Richard Purdie <rpurdie@rpsys.net>
[Fixed cocinelle warnings]
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>