nvmem: Add a simple NVMEM framework for consumers

This patch adds just consumers part of the framework just to enable easy
review.

Up until now, nvmem drivers were stored in drivers/misc, where they all
had to duplicate pretty much the same code to register a sysfs file,
allow in-kernel users to access the content of the devices they were
driving, etc.

This was also a problem as far as other in-kernel users were involved,
since the solutions used were pretty much different from on driver to
another, there was a rather big abstraction leak.

This introduction of this framework aims at solving this. It also
introduces DT representation for consumer devices to go get the data they
require (MAC Addresses, SoC/Revision ID, part numbers, and so on) from
the nvmems.

Having regmap interface to this framework would give much better
abstraction for nvmems on different buses.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
[Maxime Ripard: intial version of the framework]
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Tested-by: Philipp Zabel <p.zabel@pengutronix.de>
Tested-by: Rajendra Nayak <rnayak@codeaurora.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>