extcon: Split out extcon header file for consumer and provider device

The extcon has two type of extcon devices as following.
- 'extcon provider deivce' adds new extcon device and detect the
   state/properties of external connector. Also, it notifies the
   state/properties to the extcon consumer device.
- 'extcon consumer device' gets the change state/properties
   from extcon provider device.
Prior to that, include/linux/extcon.h contains all exported API for
both provider and consumer device driver. To clarify the meaning of
header file and to remove the wrong use-case on consumer device,
this patch separates into extcon.h and extcon-provider.h.

[Description for include/linux/{extcon.h|extcon-provider.h}]
- extcon.h includes the extcon API and data structure for extcon consumer
  device driver. This header file contains the following APIs:
  : Register/unregister the notifier to catch the change of extcon device
  : Get the extcon device instance
  : Get the extcon device name
  : Get the state of each external connector
  : Get the property value of each external connector
  : Get the property capability of each external connector

- extcon-provider.h includes the extcon API and data structure for extcon
  provider device driver. This header file contains the following APIs:
  : Include 'include/linux/extcon.h'
  : Allocate the memory for extcon device instance
  : Register/unregister extcon device
  : Set the state of each external connector
  : Set the property value of each external connector
  : Set the property capability of each external connector

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Acked-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Acked-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>