edac: e752x: add dram scrubbing support

Add support to scrub DRAM using the e752x integrated memory scrubbing
engine.  The e7320/7520/e7525 chipsets support scrubbing at one rate while
the i3100 chipset supports a normal and fast rate.

A similar patch was originally sent back in 2008:
http://sourceforge.net/mailarchive/forum.php?thread_name=1204835866.25206.70.camel@localhost.localdomain&forum_name=bluesmoke-devel



This version has the following updates:
- Use 16-bit PCI config cycles to access MCHSCRB register
    e7320/7520/e7525 docs say register is 16bits wide, i3100 says 8.  I
    tested 16bits on the i3100 to be safe.
- Recalcuate and round actual scrub rates

The changes have been tested on an i3100-based board.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>
Signed-off-by: Doug Thompson <dougthompson@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>