[POWERPC] add of_get_mac_address and update fsl_soc.c to use it

Add function of_get_mac_address(), which obtains the best MAC address to use
from the device tree by checking various properties in order.  The order is:
'mac-address', then 'local-mac-address', then 'address'.  It skips properties
that contain invalid MAC addresses, which were probably not initialized
by U-Boot.

Update gfar_of_init() and fs_enet_of_init() in fsl_soc.c to call
of_get_mac_address().

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>