armv8: spl: Support separate stack for TPL

armv8: spl: Support separate stack for TPL

To allow a TPL and SPL to run from different addresses/memories, we
need to split setup of the TPL and SPL stacks.  To do so, we introduce
CONFIG_TPL_STACK (not listed in Kconfig) which can be used to override
the initial stack pointer for TPL.

To provide backward compatibility for existing boards, this is added
as an optional configuration item and the normal search order (i.e.
SPL_STACK, then SYS_STACK) apply if not defined.

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