drivers: rtt: Lock interrupts around RTT Write

Since RTTWriteNoLock is not thread-safe or preemptable, it is necessary
to lock interrupts around the calls to it.

Change-id: I38b7e37a0f46e77bf82c1e07d549414393e84a51
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>