omap2 clock: move SDRC autorefresh counter defines into sdrc.h

Move the SDRC autorefresh counter defines from clock.h into sdrc.h,
a more logical place for them.  Expand the comments to add more explanation
as to how those values are derived.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>