usb: chipidea: udc: add new API ci_hdrc_gadget_connect

[ Upstream commit d16ab536aad208421c5ed32cdcb01b5ab6aa1f19 ]

This API is used enable device function, it is called at below
situations:
- VBUS is connected during boots up
- Hot plug occurs during runtime

Signed-off-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Jun Li <jun.li@nxp.com>
Stable-dep-of: b7a62611fab7 ("usb: chipidea: add USB PHY event")
Signed-off-by: Sasha Levin <sashal@kernel.org>