[s390x] Add time source based on the architectural Time-of-Day clock

The S/390 architecture provides instructions to read the Time-of-Day
(TOD) clock, which increments at a well-defined rate regardless of the
underlying physical clock speed, and has an epoch that starts from
zero at the beginning of the 20th century.

Use this clock to provide both interval timing (i.e. udelay() and
currticks()) and the wall-clock time source.  For short interval
timing, we choose to save on code size by treating one millisecond as
1024 microseconds.

Signed-off-by: Michael Brown <mcb30@ipxe.org>