i2c realtime clocks now work

[Ian Lepore]

In the past it wasn't possible to use i2c rtc chips/modules on some arm
boards because the i2c driver would trigger a witness assertion about
sleeping while holding locks.  The only option was to use sysctl to set
machdep.disable_rtc_set=1 if your board had an i2c clock.

As of r320902 that restriction is removed and i2c clocks should work on
all systems.

-- Ian