spl: Introduce spl_board_prepare_for_optee() hook

OP-TEE requires some particular setup, which is not needed for linux
or other payloads. Add a hook for platform-specific code to perform
any OP-TEE related configuration and initialization.

A weak function is used because it is symmetrical to other
spl_board_prepare_for_*() implementations. A solution to avoid the use
of weak functions would trivially apply to all these implementations.
However, re-designing this is beyond the scope of this patch.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>