MFC r315589, r315591, r316659, r316661:
Eliminate unnecessary read/modify/write sequences during eventtimer setup.
Replace the hard-coded way-too-small minimum event timer period with a value
calculated at runtime based on how long it takes to set up an event in
hardware. This fixes the intermittant 1-minute hang at boot on imx5
systems, and also the occasional oversleeping while running. It doesn't
affect imx6 systems, which use different hardware for eventtimers.
Remove some old interrupt handling workaround code from the pre-INTRNG days.
At this point, INTRNG is not going away (the option may go away, but the
code is not), so we no longer need code to support workarounds that handled
the lack of INTRNG functionality.
Update the code that compensates for the lack of a GPC interrupt controller
driver for imx6. Some newer dts source puts the GIC node at the root
instead of under /soc, so look in both places. Also, sometimes the GIC
node doesn't list itself as its own interrupt-parent, allow that too.