sunxi: Move INITIAL_USB_SCAN_DELAY to driver Kconfig

This option is used only by the phy-sun4i-usb driver, which does not
inherently depend on the ARM architecture.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>