sunxi: Kconfig: rework PHY_USB_SUN4I selection

At the moment we use "select" in each Allwinner SoC's Kconfig section to
include the USB PHY driver in the build. This means it cannot be disabled
via Kconfig, although USB is not really a strictly required core
functionality, and a particular board might not even include USB ports.

Rework the Kconfig part by removing the "select" lines for each SoC's
section, and instead letting it default to "y" in the PHY driver section
itself. We use "depends on !" to exclude the few SoCs we don't support
(yet). The Allwinner V3s does not enable USB (PHY) support at the moment,
even though it should work: let the PHY default to "n" to keep the
current behaviour.

Also the MUSB USB driver directly calls some functions from the PHY
driver, so let the former depend on the PHY driver.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Tested-by: Sam Edwards <CFSworks@gmail.com>