omap4_prcm: Delay the frequencies read check

Same as r333305, with Linux 4.17 dts the compatible for the prcm added
'simplebus', it 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 omap_scm one to be already
attached. omap_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 pandaboard

Approved by: re (rgrimes)