USB: ehci-omap: Don't select any PHY driver

Don't select NOP_USB_XCEIV. Instead, board config
must select USB_PHY and the appropriate PHY driver.

Also add a hint in Kconfig so that users enabling
this driver manually enable the right PHY drivers as well.

Gets rid of the below warnings when USB_EHCI_HCD_OMAP
is enabled.

warning: (USB_EHCI_HCD_OMAP) selects NOP_USB_XCEIV which has unmet direct dependencies (USB_SUPPORT && USB_PHY)
warning: (USB_EHCI_HCD_OMAP) selects NOP_USB_XCEIV which has unmet direct dependencies (USB_SUPPORT && USB_PHY)

Signed-off-by: Roger Quadros <rogerq@ti.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>