[POWERPC] Generalise device_node flag interface

The struct device_node currently has a _flags variable, although
it's only used for one flag - OF_DYNAMIC.  Generalise the flag
accessors so we can use them with other flags in future.

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