[POWERPC] Fix declaration of pcibios_free_controller

pcibios_free_controller() is now available for both 32 and 64 bits
but the header only declares it for 64 bits.  This moves the
declaration down next to the pcibios_alloc_controller() one.

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