drivers uart_nrfx: Break infinite loops for simulation

While waiting for the UART to be ready in ISR
mode, for simulation only, add a tiny delay per
iteration of the busy wait loops to allow
time to pass.
This Z_SPIN_DELAY is an empty macro for any
other target than simulation.

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