PHY: Ingenic: fix unconditional build of phy-ingenic-usb

Currently drivers/phy/ingenic/Makefile adds phy-ingenic-usb to targets
not depending on actual Kconfig symbol CONFIG_PHY_INGENIC_USB, so this
driver always gets built[-in] on every system.
Add missing dependency.

Fixes: 31de313d

 ("PHY: Ingenic: Add USB PHY driver using generic PHY framework.")
Signed-off-by: Alexander Lobakin <alobakin@pm.me>
Link: https://lore.kernel.org/r/20201222131021.4751-1-alobakin@pm.me
Signed-off-by: Vinod Koul <vkoul@kernel.org>