dt-bindings: i2c-mpc: Document interrupt property as required

All of the in-tree device-trees that use the one of the compatible
strings from i2c-mpc.c supply an interrupts property. Make this property
mandatory to aid refactoring the driver.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>