readclock: code clean-up, add -q, manpage updates

 - Simplify the message passing between readclock and the driver.

 - Add a -q command line option to suppress writing to STDOUT. This
 cuts down on the noise when readclock is called early in the boot
 sequence and a secondary RTC driver (ex TPS95650) isn't up yet.

 - Update the man page to be less i386 centric and add details about
 the new -q option.

Change-Id: If8d7c50a217ca98c1e9fae0ca92521e2e7c893e4
readclock.drv: add support for the TPS65950 RTC

On the BeagleBoard-xM, the RTC is located on the Power Management IC
(PMIC). To keep things consistent, access to the PMIC's RTC is done
through the readclock driver. The readclock driver forwards the request
on to the TPS65950 driver which does the work of manipulating the
registers on the chip.

Change-Id: I53cefbb59c5a9ab87fab90df3cc1a75a6e430f58