sm: Rework the Kconfig logic here

sm: Rework the Kconfig logic here

The symbol "SM" is a library symbol and should not be prompted for. It
should be selected by the drivers that use it. In this case we need to
add a SANDBOX_SM symbol for the sandbox driver. The meson SM driver
cannot build on other platforms, so add the appropriate dependency.

Reviewed-by: Peter Robinson <pbrobinson@gmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>