soc: actions: Update SPS help text for S700

Commit 3ad85b08 (soc: actions: sps: Add S700)
added S700 support to the SPS driver but forget to update Kconfig help.

Add missing S700 mention, in preparation for further SoCs.

Fixes: 3ad85b08 ("soc: actions: sps: Add S700")
Reported-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Andreas Färber <afaerber@suse.de>