OMAP3: Overo: Update GPMC timing for ethernet chip

The existing timing does not quite meet the minimum requirements
in the LAN9221 datasheet. The timing in this patch solves problems
noticed on some parts. The patch also combines the CS configuration
for the overo and igep0020 boards per request.

Signed-off-by: Philip Balister <philip@opensdr.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>