spl: Correct address in spl_relocate_stack_gd()

During the Kconfig conversion one of the changes was missed.
CONFIG_SPL_STACK_R should be CONFIG_SPL_STACK_R_ADDR since we want the
address.

Reported-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Simon Glass <sjg@chromium.org>