[TG3]: Speed up SRAM access

Speed up SRAM read and write functions if possible by using MMIO
instead of config. cycles. With this change, the post reset signature
done at the end of D3 power change must now be moved before the D3
power change.

IBM reported a problem on powerpc blades during ethtool self test
that was caused by the memory test taking excessively long. Config.
cycles are very slow on powerpc and the memory test can take more
than 10 seconds to complete using config. cycles. As a result, NETDEV
WATCHDOG can be triggered during self test and the chip can end up in
a funny state.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>