drivers: hwinfo: Add device EUI64 ID support

Some devices, mostly with radio support, have an EUI64 ID (unique and
attributed by IEEE), in addition to the standard device ID.

Add support for reading it through the hwinfo API. As the size is always
the same (8 bytes), there is no need to pass the size in the function,
nor return the number of bytes copied.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>