scripts: genpinctrl: support non-Git pin data folder

This commit adds error checking to the call to `git rev-parse HEAD`,
to ensure genpinctrl does not crash when the pin data folder is not
a Git repository. In such cases, "<unknown commit>" is written to
the pinctrl README file.

Signed-off-by: Mathieu Choplain <mathieu.choplain@st.com>