OMAP3 Move declaration of gpmc_cfg.

Every omap3 board config file declared the global variable gpmc_cfg.
This changes moves the declaration to a better location in the
arch dependent header file cpu.h.

Signed-off-by: Tom Rix <Tom.Rix@windriver.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>