extcon: Add devm_extcon_dev_allocate/free to manage the resource of extcon device

This patch add device managed devm_extcon_dev_{allocate,free} to automatically
free the memory of extcon_dev structure without handling free operation.

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Reviewed-by: Felipe Balbi <balbi@ti.com>