powerpc: Move FW feature probing out of pseries probe()

We move the function itself to pseries/firmware.c and call it along
with almost all other flat device-tree parsers from early_init_devtree()

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
[mpe: Move #ifdefs into the header by providing pseries_probe_fw_features()]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>