[POWERPC] Split initrd logic out of early_init_dt_scan_chosen() to fix warning

If CONFIG_BLK_DEV_INITRD is not defined the prop variable in
early_init_dt_scan_chosen() is unused, causing a compiler warning.

So split the initrd logic into a separate function, allowing us to
declare prop only when we need it.

Built for both cases and booted with an initrd.

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