nios2 - adjust gbl data off to account for bd_info

 * Adjust the GBL_DATA_OFFSET to account for the fact that we'll be
   using the space between that offset and the start of the malloc
   region to allocate both a gd structure and a board info structure.

 * This fixes a memory corruption bug due to overlap of the malloc
   region and the bd_info structure.

Signed-off-by: Alex Hornung <alex@alexhornung.com>
Signed-off-by: Thomas Chou <thomas@wytron.com.tw>