drm: sti: fix typos in stih4xx binding

Fix typos in the st,stih4xx binding, in particular replacing
"pinctrl-name" by "pinctrl-names".

Fix minor typos in the descriptions too.

Signed-off-by: Michael Opdenacker <michael.opdenacker@free-electrons.com>
Signed-off-by: Rob Herring <robh@kernel.org>