generic: platform/mikrotik: add NVMEM layout driver

generic: platform/mikrotik: add NVMEM layout driver

Currently, information from MikroTik hard_config is only available via
sysfs, meaning that we have to rely on userspace to for example setup MACs.

So, lets provide a basic NVMEM layout based driver to expose the same cells
as sysfs driver exposes.

Do note that the we dont extract the WLAN caldata and BDF-s at this point.

Reviewed-by: Thibaut VARÈNE <hacks@slashdirt.org>
Link: https://github.com/openwrt/openwrt/pull/15665
Signed-off-by: Robert Marko <robimarko@gmail.com>