[POWERPC] pmac_feature_call checks platform

This patch makes sure that a caller of pmac_call_feature() won't try
to call into ppc_md.feature_call of another platform, which might
happen if some powermac drivers are loaded on non-powermac machines.

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