[POWERPC] Merge 32 and 64 bits pcibios_enable_device

This merge the two implementations, based on the previously
fixed up 32 bits one.  The pcibios_enable_device_hook in ppc_md
is now available for ppc64 use.  Also remove the new unused
"initial" parameter from it and fixup users.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>