extcon: palmas: Use devm_extcon_dev_allocate for extcon_dev

This patch use devm_extcon_dev_allocate() to simplify the memory control
of extcon device.

Cc: Graeme Gregory <gg@slimlogic.co.uk>
Cc: Kishon Vijay Abraham I <kishon@ti.com>
Cc: Felipe Balbi <balbi@ti.com>
Cc: Samuel Ortiz <sameo@linux.intel.com>
Cc: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Felipe Balbi <balbi@ti.com>
Tested-by: Felipe Balbi <balbi@ti.com>