powerpc/configs: Enable CONFIG_USB_XHCI_HCD by default

Recent versions of QEMU provide a XHCI device by default these
days instead of an old-fashioned OHCI device:

 https://git.qemu.org/?p=qemu.git;a=commitdiff;h=57040d451315320b7d27

So to get the keyboard working in the graphical console there again,
we should now include XHCI support in the kernel by default, too.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Acked-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>