ethernet: Extend device_get_mac_address() to use NVMEM

ethernet: Extend device_get_mac_address() to use NVMEM

A lot of modern SoC have the ability to store MAC addresses in their
NVMEM. So extend the generic function device_get_mac_address() to
obtain the MAC address from an nvmem cell named 'mac-address' in
case there is no firmware node which contains the MAC address directly.

Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20250912140332.35395-3-wahrenst@gmx.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>