mfd: cpcap: Add minimal support

Many Motorola phones like droid 4 are using a custom PMIC called CPCAP
or 6556002. We can support it's core features quite easily with regmap_spi
and regmap_irq.

The children of cpcap, such as regulators, ADC and USB, can be just regular
device drivers and defined in the dts file. They get probed as we call
of_platform_populate() at the end of our probe, and then the children
can just call dev_get_regmap(dev.parent, NULL) to get the regmap.

Cc: devicetree@vger.kernel.org
Cc: Marcel Partap <mpartap@gmx.net>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Michael Scott <michael.scott@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>