hsi: Only descend into hsi directory when CONFIG_HSI is set

When CONFIG_HSI is not set make will still descend into the hsi
directory but nothing will be built. This produces unneeded build
artifacts and messages in addition to slowing the build. Fix this here.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>