dt-bindings: i2c: aspeed: drop redundant multi-master

'multi-master' property is defined by core i2c-controller schema in
dtschema package, so binding which references it and has
unevaluatedProperties:false, does not need to mention it.  It is
completely redundant here.

Suggested-by: Andi Shyti <andi.shyti@kernel.org>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>