mfd: add PCAP driver

The PCAP Asic as present on EZX phones is a multi function device with
voltage regulators, ADC, touch screen controller, RTC, USB transceiver,
leds controller, and audio codec.

It has two SPI ports, typically one is connected to the application
processor and another to the baseband, this driver provides read/write
functions to its registers, irq demultiplexer and ADC
queueing/abstraction.

This chip is used on a lot of Motorola phones, it was manufactured by TI
as a custom product with the name PTWL93017, later this design evolved
into the ATLAS PMIC from Freescale (MC13783).

Signed-off-by: Daniel Ribeiro <drwyrm@gmail.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>