[POWERPC] Setup RTAS values earlier, to enable rtas_call() earlier

Althought RTAS is instantiated when we enter the kernel, we can't actually
call into it until we know its entry point address. Currently we grab that
in rtas_initialize(), however that's quite late in the boot sequence.

To enable rtas_call() earlier, we can grab the RTAS entry etc. values while
we're scanning the flattened device tree. There's existing code to retrieve
the values from /chosen, however we don't store them there anymore, so remove
that code.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>