rockchip: rk3399: derive ethaddr from cpuid

Generate a MAC address based on the cpuid available in the efuse
block: Use the first 6 byte of the cpuid's SHA256 hash and set the
locally administered bits. Also ensure that the multicast bit is
cleared.

The MAC address is only generated and set if there is no ethaddr
present in the saved environment.

This is based off of Klaus Goger's work in 8adc9d

Signed-off-by: Rohan Garg <rohan.garg@collabora.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>