docs: extend deprecation policy

Our process documentation already mentions that if a platform is no
longer maintained, it is best to deprecate it to keep the project's
source tree clean and healthy.

The same argument stands for drivers or library interfaces so extend
this policy to those.

Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
Change-Id: Ieb235d6a1fb089343e0e1e3e5f36067552f2f8f0
1 file changed