omap4: board: change global data pointer to file scope

DECLARE_GLOBAL_DATA_PTR is currently defined within the scope
of function while it is a global pointer. Change the scope of
definition to replicate it's global scope. This seems to help
gcc 4.5 optimizations as well.

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>