phy: allwinner: allow compile testing

Some allwinner specific drivers can be built for testing purposes
on non-sunxi configurations, which then results in a harmless
warning:

WARNING: unmet direct dependencies detected for PHY_SUN6I_MIPI_DPHY
  Depends on [n]: ARCH_SUNXI [=n] && HAS_IOMEM [=y] && OF [=y] && RESET_CONTROLLER [=y]
  Selected by [y]:
  - DRM_SUN6I_DSI [=y] && HAS_IOMEM [=y] && DRM_SUN4I [=y]

Allow compile-test here as well to avoid the warning, and improve
overall build coverage.

Fixes: 5d134abf ("phy: Move Allwinner A31 D-PHY driver to drivers/phy/")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>