rtlwifi: btcoexist: Add new mini driver

A new driver in the rtlwifi family for the RTL8723BE will soon be added.
The bluetooth coexistence code for this device has been split into a separate
mini driver as it will be shared with other devices.  This commit adds the
the headers and sources, and modifies Kconfig and Makefile to configure and
build this driver.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>