dts: nordic,nrf-twim: rename non-hardware property

As a general rule devicetree properties should correspond to hardware
description or configuration.  In cases where a Zephyr driver receives
instance-specific configuration data from a devicetree property that
property should be marked as being Zephyr-specific.  Rename
concat-buf-size to zephyr,concat-buf-size to follow this guideline.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>