phy: sun9i-usb: add USB dependency

The sun9i usb phy driver calls of_usb_get_phy_mode(), which is not
available if USB is disabled:

drivers/built-in.o: In function `sun9i_usb_phy_probe':
:(.text+0x7fb0): undefined reference to `of_usb_get_phy_mode'

This adds a dependency to avoid the randconfig build errors.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 9c3b4430 ("phy: Add driver to support individual USB PHYs on sun9i")
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>