net: bgmac-platform: handle mac-address deferral

This patch is a replication of Christian Lamparter's "net: bgmac-bcma:
handle deferred probe error due to mac-address" patch for the
bgmac-platform driver [1].

As is the case with the bgmac-bcma driver, this change is to cover the
scenario where the MAC address cannot yet be discovered due to reliance
on an nvmem provider which is yet to be instantiated, resulting in a
random address being assigned that has to be manually overridden.

[1] https://lore.kernel.org/netdev/20210919115725.29064-1-chunkeey@gmail.com

Signed-off-by: Matthew Hagan <mnhagan88@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>