powerpc/512x: silence a USB Kconfig dependency warning

powerpc/512x: silence a USB Kconfig dependency warning

the PPC_MPC512x config automatically selected USB_EHCI_BIG_ENDIAN_*
switches, which made Kconfig warn about "unmet direct dependencies":

  scripts/kconfig/conf --silentoldconfig Kconfig
  warning: (PPC_MPC512x && 440EPX) selects USB_EHCI_BIG_ENDIAN_DESC which has unmet direct dependencies (USB_SUPPORT && USB && USB_EHCI_HCD)
  warning: (PPC_MPC512x && PPC_PS3 && PPC_CELLEB && 440EPX) selects USB_EHCI_BIG_ENDIAN_MMIO which has unmet direct dependencies (USB_SUPPORT && USB && USB_EHCI_HCD)
  warning: (PPC_MPC512x && 440EPX) selects USB_EHCI_BIG_ENDIAN_DESC which has unmet direct dependencies (USB_SUPPORT && USB && USB_EHCI_HCD)
  warning: (PPC_MPC512x && PPC_PS3 && PPC_CELLEB && 440EPX) selects USB_EHCI_BIG_ENDIAN_MMIO which has unmet direct dependencies (USB_SUPPORT && USB && USB_EHCI_HCD)

make the selected entries additionally depend on USB_EHCI_HCD which
silences the warning

Signed-off-by: Gerhard Sittig <gsi@denx.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>