powerpc/mpc85xx: Improve disabling of SPE instructions

powerpc/mpc85xx: Improve disabling of SPE instructions

Specifying -mspe=no also disables usage of SPE instructions. It is
documented in "[PATCH,rs6000] make -mno-spe work as expected" email:
http://gcc.gnu.org/ml/gcc-patches/2008-04/msg00311.html

So replace -mspe=yes by -mspe=no, so make it clear that u-boot has to be
compiled without SPE instructions.

Linux kernel contains following Makefile code to achieve it:

    # No SPE instruction when building kernel
    # (We use all available options to help semi-broken compilers)
    KBUILD_CFLAGS += $(call cc-option,-mno-spe)
    KBUILD_CFLAGS += $(call cc-option,-mspe=no)

Do same for U-Boot.

Signed-off-by: Pali Rohár <pali@kernel.org>