doc: document the pwm command

Signed-off-by: Emil Kronborg <emil.kronborg@protonmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 1f6518b..6a218c4 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -93,6 +93,7 @@
    cmd/pinmux
    cmd/printenv
    cmd/pstore
+   cmd/pwm
    cmd/qfw
    cmd/read
    cmd/reset