drm/bridge: Improve the help text for DRM_ANALOGIX_ANX78XX

Improve the help text for DRM_ANALOGIX_ANX78XX by adding the missing
"power" word.

After this change the help text matches with the ANX7814
product description from the Analogix website:

https://www.analogix.com/en/products/convertersbridges/anx7814

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190722194049.20761-1-festevam@gmail.com