PM changes for am335x and am437x

This series adds support for am437x RTC-only mode in suspend. In the
RTC-only mode suspend, everything is shut down except the RTC. This
makes the power consumption very low for suspend mode.

To support RTC-only mode, we need to export omap_rtc_power_off_program()
from the rtc driver and improve PM code to save and restore the wkup
domain context. As RTC-only mode depends on the device being wired
properly for things like memory, we need to also check for the machine
type before we allow it. We also need to run DDR3 hardware leveling on
resume.

Note that there is a trivial merge conflict between the RTC branch
and these changes where the RTC branch makes tm2bcd() a void function
and the error handling parts can be just dropped.
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