ehci-platform: Change compatible string from usb-ehci to generic-ehci

The initial versions of the devicetree enablement patches for ehci-platform
used "ehci-platform" as compatible string. However this was disliked by various
reviewers because the platform bus is a Linux invention and devicetree is
supposed to be OS agnostic. After much discussion I gave up, added a:
"depends on !PPC_OF" to Kconfig to avoid a known conflict with PPC-OF platforms
and went with the generic usb-ehci as requested.

In retro-spect I should have chosen something different, the dts files for many
existing boards already claim to be compatible with "usb-ehci", ie they have:

	compatible = "ti,ehci-omap", "usb-ehci";

In theory this should not be a problem since the "ti,ehci-omap" entry takes
presedence, but in practice using a conflicting compatible string is an issue,
because it makes which driver gets used depend on driver registration order.

This patch changes the compatible string claimed by ehci-platform to
"generic-ehci", avoiding the driver registration / module loading ordering
problems, and removes the "depends on !PPC_OF" workaround.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Tested-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>