armv8: TPL_STACK will always be defined, so test CONFIG_TPL_NEEDS_SEPARATE_STACK

Now that TPL_STACK has been moved off the whitelist (ok, I'm lying:
the 'moving off the whitelist' part comes in once moveconfig
runs... which will be a few commits down the line) and added to
Kconfig, we need to test CONFIG_TPL_NEEDS_SEPARATE_STACK to see
whether the value from TPL_STACK should be used or whether we try to
inherit whatever SPL uses.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>