drivers serial nrfx: Speed up for simulation

Increase the busy wait time granularity while runing in simulation
for poll mode. In this mode, the driver spends a *lot* of time
waiting for the UART to be done. The smallest frame time is
~10µs @ 1Mbps, so busywaiting in very small increments
is very wastefull as we keep shuting down and turning on the simulated
MCU.
Let's increase the busy wait time increments of the driver to 3 micros,
which should not change much the behaviour.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>