treewide: convert mtd-mac-address-increment* to generic implementation

Rework patch 681-NET-add-mtd-mac-address-support to implement
only the function to read the mac-address from mtd.
Generalize mtd-mac-address-increment function so it can be applied
to any source of of_get_mac_address.
Rename any mtd-mac-address-increment to mac-address-increment.
Rename any mtd-mac-address-increment-byte to mac-address-increment-byte.

This should make simplify the conversion of target to nvmem implementation.

Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>