scripts: locked pyocd version to 0.12.0 in requirements.txt

The recently released version 0.13.0 of pyocd does not install cleanly
in linux and prevents installation of requirements.txt. Locking pyocd to
version 0.12.0 is a temporary workaround until problem is solved by
pyocd maintainers.

Signed-off-by: Mikkel Jakobsen <mikkel.aunsbjerg@prevas.dk>