Some more were added since the cleanup pass in June 2018. See e.g.
commit 2d50da70a1 ("drivers: ipm: Kconfig: Remove redundant 'default n'
properties") for a motivation. It also avoids people wondering whether
or not they need to put in 'default n'.
Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>