layerscape: Fix multiple bugs in of_get_mac_address() changes

The change which backported the of_get_mac_address() change broke some
patches in the layerscape target so the patches did not apply any more.

This commit makes them apply again and also fixes some other problems
related to this change.

Fixes commit 91a52f22 ("treewide: backport support for nvmem on non platform devices")
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>