at91sam9260/9263: add back up for the rst(reset controller).

On the boards at91sam9260ek, at91sam9263ek and afed9260, the rstc register was
set to 0 after being set to 500 ms for the PHY reset.
Do backup the old reset length and restore it after the MACB initialisation.

Signed-off-by: Sedji Gaouaou <sedji.gaouaou@atmel.com>
Signed-off-by: Stelian Pop <stelian@popies.net>