src/southbridge: add IS_ENABLED() around Kconfig symbol references

Change-Id: Ie965cbcf7f7b6f6c9e9a69e2a1ff0ba491246cbe
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/20353
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
15 files changed