devicetree: Add msi to the vendor-prefix list

Document the the "msi" (Micro-Star International Co. Ltd.) vendor prefix
which is used in sun6i-a31s-primo81.dts.

Signed-off-by: Karsten Merker <merker@debian.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>