Zoom2 Fix serial gpmc setup

The offset to the chip select is incorrect.

The change 187af954,

omap3: embedd gpmc_cs into gpmc config struct

introduced a problem with the serial gpmc setup.

This patch reverts the chip select to its previous value.

The symptoms of this problem are that the Zoom2
currently hangs.

This was run tested on Zoom2.

Signed-off-by: Tom Rix <Tom.Rix@windriver.com>