common/board_f: remove XTRN_DECLARE_GLOBAL_DATA_PTR dead code

common/board_f: remove XTRN_DECLARE_GLOBAL_DATA_PTR dead code

The XTRN_DECLARE_GLOBAL_DATA_PTR declarations in ppc code are permanently
commented out, so there are no users for this macro:
 #if 1
   #define DECLARE_GLOBAL_DATA_PTR   register volatile gd_t *gd asm ("r2")
 #else /* We could use plain global data, but the resulting code is bigger */
   #define XTRN_DECLARE_GLOBAL_DATA_PTR   extern
   #define DECLARE_GLOBAL_DATA_PTR     XTRN_DECLARE_GLOBAL_DATA_PTR \
                                       gd_t *gd
 #endif

Remove all references to this macro, but add a documentation note regarding
the possibility of using plain global data for the GD pointer.

Signed-off-by: Ovidiu Panait <ovpanait@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>