ARM: tegra: timer: Remove 32k clocksource

ARM: tegra: timer: Remove 32k clocksource

The 32k clocksource is never used, because it is always registered
at the same time as the us clocksource, which has a higher rating.
Using the 32k clocksource would cause a race condition with the
tegra_rtc driver on the RTC shadow registers.  Remove the 32k
clocksource, add a replacement tegra_read_rtc_ms function for the
suspend code to track suspend time, and add a comment on
read_persistent_clock that it should never be called while the
tegra_rtc driver is running.  It is currently only called from
timekeeping init, suspend, and resume.

Change-Id: Ic964578e9923b0f0338959f75de1ac170f850337
Signed-off-by: Colin Cross <ccross@android.com>