Switch the timer unit from 24MHz ticks to microseconds

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Make the client generate a square wave at 50kHz

to see how the FIQ behave at higher frequency. This will generate the
output on the GPIO line exported on the +Y connector

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>