Conditionally removed PMP documentation

Created by: Silabs-ArjanB

Conditionally removed PMP documentation as CV32E40P does not have a PMP.

For future chips, text can be rendered with PMP material as well by using '-t PMP', e.g. sphinx-build -b html -t PMP "source" "build"

Signed-off-by: Arjan Bink Arjan.Bink@silabs.com