am335x_prcm: Delay the frequencies read check

With Linux 4.17 dts the compatible for the prcm added 'simplebus' we mean
that the simplebus driver will attach to it at the BUS_PASS_BUS pass.
Change the pass for the prcm driver to be at BUS_PASS_BUS so we will win
the attach.
This introduce a problem as this driver needs the ti_scm one to be already
attached. ti_scm also attach at BUS_PASS_BUS but after the prcm one as it is
after in the dtb and the simplebus driver simpy walk the tree to attach it's
children.
Use the bus_new_pass method to defer the frequencies read at BUS_PASS_TIMER.
This fixes booting on BeagleBone*

Reported by: many