[POWERPC] Make EMAC use generic DCR access methods

This patch makes the EMAC driver use the new DCR access methods. It
doesn't yet uses dcr_map() and thus still only work with real DCRs.
This will be fixed in a later patch

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>