dt-bindings: i2c: opencores: Add missing type for "regstep"

"regstep" may be deprecated, but it still needs a type.

Fixes: 8ad69f49

 ("dt-bindings: i2c: convert ocores binding to yaml")
Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Peter Korsgaard <peter@korsgaard.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Andi Shyti <andi.shyti@kernel.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>