i2c: mpc: make interrupt mandatory and remove polling code

All the in-tree dts files that use one of the compatible strings from
i2c-mpc.c provide an interrupt property. By making this mandatory we
can simplify the code.

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