dt-bindings: panel: raydium,rm67191: Add missing type to 'video-mode'

'video-mode' is missing a type definition and is not a common property. The
type is 'uint32'.

Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220719215201.1877997-1-robh@kernel.org