add phy_enable_prop_idx() to work with phys under props other than "phy".

add phy_enable_prop_idx() to work with phys under props other than "phy".

eg, the snps,dwc3 device tree bindings say that it uses a usb2 and
usb3 phy, and they can be listed either "usb2-phy" and "usb3-phy"
under the standard "phys" and "phy-names" properties supported by
phy_enable(), or as slots 0 and 1 under a "usb-phy" properties. the
latter would be supported by phy_enable_idx(), but it hardcodes
"phys" as the property it looks at. phy_enable_prop_idx() is the
same as phy_enable_prop_idx, but it lets you specify which property
you're indexing into.

ok kettenis@