usb: chipidea: imx: change hsic power regulator as optional

[ Upstream commit 7d5ec335f94e74e885ca2f6c97a3479fe9fe3b15 ]

Not every platform needs this regulator.

Signed-off-by: Peter Chen <peter.chen@nxp.com>
Stable-dep-of: 8c531e0a8c2d ("usb: chipidea: ci_hdrc_imx: implement usb_phy_init() error handling")
Signed-off-by: Sasha Levin <sashal@kernel.org>