powerpc and sparc: Introduce dev_archdata node accessors

The name of the device_node field differ across the platforms, so we
have to implement inlined accessors.  This is needed to avoid ugly
#ifdef in the generic code.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Acked-by: David S. Miller <davem@davemloft.net>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Paul Mackerras <paulus@samba.org>