mfd: Add ADP5520/ADP5501 driver

Base driver for Analog Devices ADP5520/ADP5501 MFD PMICs

Subdevs:
LCD Backlight   : drivers/video/backlight/adp5520_bl.c
LEDs            : drivers/led/leds-adp5520.c
GPIO            : drivers/gpio/adp5520-gpio.c (ADP5520 only)
Keys            : drivers/input/keyboard/adp5520-keys.c (ADP5520 only)

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Bryan Wu <cooloney@kernel.org>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>