secure_storage: swap `select` to `depends on`

Mixing `select` and `depends on` is a common source of Kconfig
dependency loops and should be avoided. Both `ZMS` and `SETTINGS` are
more commonly used with `depends on` rather than `select`.

The usage here also contradicts the Zephyr best practices guide for
`select`:
 * Avoid selecting symbols with prompts or dependencies.
   Prefer depends on.

Signed-off-by: Jordan Yates <jordan@embeint.com>