MAINTAINERS: add keyword for devicetree overlay notifiers

Devicetree overlay notifiers have a chance to potentially get
pointers into the overlay unflattened devicetree and overlay FDT.
The only protection against these pointers being accessed after
the underlying data has been released by kfree() is by source
code review of patches.  Add a keyword line to the devicetree
overlay maintainers entry to try to catch overlay notifier
related patches.

The keyword line is added to the devicetree overlay entry instead
of the devicetree entry so that not all maintainers will receive
the additional review traffic.  Add Frank Rowand (already a
maintainer in the devicetree entry) so that he will receive
the additional review traffic.

Signed-off-by: Frank Rowand <frank.rowand@sony.com>
Signed-off-by: Rob Herring <robh@kernel.org>