board: dhelectronics: Implement common MAC address functions

This is a starting point for unifying duplicate code in the DH board
files. The functions for setting up MAC addresses are very similar for
the i.MX6, i.MX8 and stm32mp1 based boards.

All pre-existing implementations follow the same logic:

(1) Check if ethaddr is already set in the environment
(2) If not, try to get it from a board specific location (e.g. fuse)
(3) If not, try to get it from eeprom

After this commit, (1) and (3) are implemented as common functions,
ready to be used by board specific files.

Furthermore there is an implementation of (2) for imx based boards.

Signed-off-by: Philip Oberfichtner <pro@denx.de>
Reviewed-by: Marek Vasut <marex@denx.de>