mpc85xx/u-boot*.lds: remove _GLOBAL_OFFSET_TABLE_ definition

mpc85xx/u-boot*.lds: remove _GLOBAL_OFFSET_TABLE_ definition

In binutils-2.25, the _GLOBAL_OFFSET_TABLE_ symbols defined by PROVIDE in
u-boot.lds overrides the linker built-in symbols
(https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commitdiff;
h=b893397a4b1316610f49819344817715e4305de9),
so the linker is treating _GLOBAL_OFFSET_TABLE_ as a definition into the
.reloc section.

To align with the change of binutils-2.25, the _GLOBAL_OFFSET_TABLE_ symbol
should not be defined in sections, and the symbols in linker generated .got
section should be used(https://sourceware.org/ml/binutils/2008-09/
msg00122.html)

Fixed the following build errors with binutils-2.25:
| powerpc-poky-linux-gnuspe-ld.bfd: _GLOBAL_OFFSET_TABLE_ not defined in
linker created .got

Signed-off-by: Zhenhua Luo <zhenhua.luo@freescale.com>
Signed-off-by: Prabhakar Kushwaha <prabhakar@freescale.com>