drm/bridge/synopsys: dsi: Allow VPG to be enabled via debugfs

The Synopsys MIPI DSI IP contains a video test pattern generator which
is helpful in debugging video timing with connected displays.
Add a debugfs directory containing files which allow the VPG to be
enabled and disabled, and its orientation to be changed.

Signed-off-by: Matt Redfearn <matt.redfearn@thinci.com>
Tested-by: Yannick Fertré <yannick.fertre@st.com>
Reviewed-by: Philippe Cornu <philippe.cornu@st.com>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190430081646.23845-1-matt.redfearn@thinci.com