net: openthread: add kconfigs to change values in zephyr port

This commit adds additional Kconfigs that allow for changing
configuration values for the Zephyr port in OpenThread.

Those values are:
- number of the internal OT message buffers
- number of the state change callbacks
- number of the EID-to-RLOC cache entries
- size of the NCP buffer

Signed-off-by: Rafał Kuźnia <rafal.kuznia@nordicsemi.no>