ARM: SPL: do not set gd again

Just before calling board_init_f, crt0.S has already
reserved space for the initial gd on the stack. There
should be no need to allocate it again.

cc: Albert ARIBAUD <albert.u.boot@aribaud.net>
Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>