stmmac: dwmac-sti: Pass sysconfig register offset via syscon dt property.

Based on Arnds review comments here https://lkml.org/lkml/2014/11/13/161

,
we should not be mixing address spaces in the reg property like this driver
currently does. This patch updates the driver, dt docs and also the existing
dt nodes to pass the sysconfig offset in the syscon dt property.

This patch breaks DT compatibility! But this platform is considered WIP,
and is only used by a few developers who are upstreaming support for it.
This change has been done as a single atomic commit to ensure it is
bisectable.

Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>