UPSTREAM: extcon: link devres into core module

UPSTREAM: extcon: link devres into core module

Splitting the resource-managed functions into a separate module
means that the extcon core now fails to build because the internal
"extcon_dev_allocate" symbol is not exported:

ERROR: extcon_dev_allocate [drivers/extcon/devres.ko] undefined!

My guess is that the intention was not to have two separate
modules (which could be fixed by adding an export, plus the
normal MODULE_AUTHOR/MODULE_LICENSE/... fields), but have two
source files in the same module.

This fixes the Makefile accordingly, making the name of the
module extcon_core.ko, which is created from building both
extcon.c and devres.c.

Fixes: b225d00f3ad2 ("extcon: Split out the resource-managed functions from extcon core")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
(cherry picked from commit af61f96109b73fefbe0589c320d2219567f4f660
  git://git.kernel.org/pub/scm/linux/kernel/git/chanwoo/extcon.git extcon-next)

Change-Id: I8a319f46437cd3d6ce98a4f6ac5d1c56930c9c76
Signed-off-by: Chris Zhong <zyw@rock-chips.com>