generic_board: reduce the redundancy of gd_t struct members

This commit refactors common/board_f.c and common/board_r.c
in order to delete the dest_addr and dest_addr_sp from
gd_t struct.

As mentioned as follows in include/asm-generic/global_data.h,

  /* TODO: is this the same as relocaddr, or something else? */
  unsigned long dest_addr;        /* Post-relocation address of U-Boot */

dest_addr is the same as relocaddr.
Likewise, dest_addr_sp is the same as start_addr_sp.

It seemed dest_addr/dest_addr_sp was used only as a scratch variable
to calculate relocaddr/start_addr_sp, respectively.

With a little refactoring, we can delete dest_addr and dest_addr_sp.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Simon Glass <sjg@chromium.org>