common/board_f: implement type casting for gd structure

In case of global data structure defined as "register volatile" compiler
throws an warning about incorrect type used:
 --->8---
 common/board_f.c: In function "board_init_f_r":
 common/board_f.c:1073:2: warning: passing argument 1 of "&board_init_r
 +(sizetype)gd->reloc_off" discards "volatile" qualifier from pointer
 target type [enabled by default]
  (board_init_r + gd->reloc_off)(gd, gd->relocaddr);
  ^
 common/board_f.c:1073:2: note: expected "struct gd_t *" but argument is
 of type "volatile struct gd_t *"
 --->8---

An obvious fix is manual casting to "gd_t *".

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@ti.com>
Acked-by: Simon Glass <sjg@chromium.org>