ipq806x: Fix default MAC addresses on MR42 by moving to nvmem-layout

ipq806x: Fix default MAC addresses on MR42 by moving to nvmem-layout
Partial, single-target update extracted from https://github.com/openwrt/openwrt/commit/d264d3a6

The previous `mac-address-increment` is deprecated, and in particular on
this target means that the kernel is unable to read the MAC address,
causing the system to boot with a new random MAC address each time.

Fixes: https://github.com/openwrt/openwrt/issues/13992
Signed-off-by: Rosen Penev <rosenp@gmail.com>
[rafal.boni@gmail.com: single-target-specific backport from larger change]
Signed-off-by: Rafal Boni <rafal.boni@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/16858
Signed-off-by: Robert Marko <robimarko@gmail.com>