phy: Restrict phy-hi6220-usb to HiSilicon arm64

The HiSilicon Hi6220 USB PHY is available in HiSilicon Hi6220 SoCs only.
Restrict it to HiSilicon arm64, unless compile-testing.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>