common: Kconfig.boot: add config SPL_FIT_RSASSA_PSS

The padding pss is only supported on u-boot and tools since
commit 2bbed3ff ("image: Use Kconfig to enable FIT_RSASSA_PSS on host")

This commit adds the config SPL_FIT_RSASSA_PSS to support
the padding pss in the SPL.

Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>
Reviewed-by: Simon Glass <sjg@chromium.org>