The PIT is not required anymore to successfully boot and may actually harm
in case preempt-rt is used because the PIT interrupt is shared.
Disable it so the TCB clocksource is used.

Link: https://lore.kernel.org/r/20191009194814.15034-1-alexandre.belloni@bootlin.comAcked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Acked-by: Alexander Dahl <ada@thorsis.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>