Fix build errors in phy-rockchip-inno-usb2.c. The driver uses
extcon interfaces so it should depend on EXTCON.

Fixes these build errors:

drivers/built-in.o: In function `rockchip_usb2phy_otg_sm_work':
phy-rockchip-inno-usb2.c:(.text+0x2bcb): undefined reference to `extcon_get_state'
phy-rockchip-inno-usb2.c:(.text+0x2cd4): undefined reference to `extcon_set_state_sync'
phy-rockchip-inno-usb2.c:(.text+0x2cec): undefined reference to `extcon_set_state_sync'
phy-rockchip-inno-usb2.c:(.text+0x2d2d): undefined reference to `extcon_get_state'
drivers/built-in.o: In function `rockchip_usb2phy_probe':
phy-rockchip-inno-usb2.c:(.text+0x31d7): undefined reference to `extcon_get_edev_by_phandle'
phy-rockchip-inno-usb2.c:(.text+0x321a): undefined reference to `devm_extcon_dev_allocate'
phy-rockchip-inno-usb2.c:(.text+0x3230): undefined reference to `devm_extcon_dev_register'
phy-rockchip-inno-usb2.c:(.text+0x375a): undefined reference to `extcon_register_notifier'

Found in linux-next but is also needed in mainline.
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: MyungJoo Ham <myungjoo.ham@samsung.com>
Cc: Chanwoo Choi <cw00.choi@samsung.com>
Cc: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>