drivers: misc: nordic_vpr_launcher: Update src/exec memory size check

Firstly, build-asserting the execution/source memory sizes to be equal
wasn't working, due to the wrong (non-inst) DT API being used.

Secondly, this assert can be relaxed so that the source memory region
only needs to have greater than or equal size to the execution region,
as VPR firmware needs to fit into execution memory first and foremost.
This will come in handy, since MRAM partitions (typical source memory)
have stricter alignment requirements than RAM regions.

Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>