Update the binding file for gpio, it is just an alignment
with kernel v5.3.
The U-Boot code example for gpio-hog (not directly linked
to binding) is moved in a new file doc/README.gpio.
[commit 21676b706e99 ("gpio: fixes for gpio-hog support")
& 'commit 4762a9988ede ("gpio: add gpio-hog support")']

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>