powerpc: Fix build failure on 44x

Fix the following build failure

drivers/built-in.o: In function `nhi_init':
nhi.c:(.init.text+0x63390): undefined reference to `ehci_init_driver'

by adding a dependency on USB_EHCI_HCD which supplies the ehci_init_driver().

Also we need to depend on USB_OHCI_HCD similarly
Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Acked-by: Alistair Popple <alistair@popple.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/platforms/44x/Kconfig

@@ -216,8 +216,8 @@ config AKEBONO
 	select IBM_EMAC_EMAC4
 	select IBM_EMAC_RGMII_WOL
 	select USB
-	select USB_OHCI_HCD_PLATFORM
-	select USB_EHCI_HCD_PLATFORM
+	select USB_OHCI_HCD_PLATFORM if USB_OHCI_HCD
+	select USB_EHCI_HCD_PLATFORM if USB_EHCI_HCD
 	select MMC_SDHCI
 	select MMC_SDHCI_PLTFM
 	select MMC_SDHCI_OF_476GTR