ENGR00288522 power: imx6-usb-charger: fix build error when build as module

ENGR00288522 power: imx6-usb-charger: fix build error when build as module

It is a library, so it can't be built as a module, besides,
it uses anatop register, it should depends on imx6 soc series.
Below is the build error message it fixes:

CC [M]  drivers/power/imx6_usb_charger.o
/home/b29397/work/projects/linux-2.6-imx/drivers/power/imx6_usb_charger.c:173:5: error: redefinition of 'imx6_usb_vbus_connect'
/home/b29397/work/projects/linux-2.6-imx/include/linux/power/imx6_usb_charger.h:70:5: note: previous definition of 'imx6_usb_vbus_connect' was here
/home/b29397/work/projects/linux-2.6-imx/drivers/power/imx6_usb_charger.c:200:5: error: redefinition of 'imx6_usb_charger_detect_post'
/home/b29397/work/projects/linux-2.6-imx/include/linux/power/imx6_usb_charger.h:74:5: note: previous definition of 'imx6_usb_charger_detect_post' was here
/home/b29397/work/projects/linux-2.6-imx/drivers/power/imx6_usb_charger.c:235:5: error: redefinition of 'imx6_usb_vbus_disconnect'
/home/b29397/work/projects/linux-2.6-imx/include/linux/power/imx6_usb_charger.h:65:5: note: previous definition of 'imx6_usb_vbus_disconnect' was here
/home/b29397/work/projects/linux-2.6-imx/drivers/power/imx6_usb_charger.c:256:5: error: redefinition of 'imx6_usb_create_charger'
/home/b29397/work/projects/linux-2.6-imx/include/linux/power/imx6_usb_charger.h:59:5: note: previous definition of 'imx6_usb_create_charger' was here
/home/b29397/work/projects/linux-2.6-imx/drivers/power/imx6_usb_charger.c:290:6: error: redefinition of 'imx6_usb_remove_charger'
/home/b29397/work/projects/linux-2.6-imx/include/linux/power/imx6_usb_charger.h:54:6: note: previous definition of 'imx6_usb_remove_charger' was here
make[3]: *** [drivers/power/imx6_usb_charger.o] Error 1
make[2]: *** [drivers/power] Error 2

Signed-off-by: Peter Chen <peter.chen@freescale.com>