DTT: Issue one-shot command on AD7414 (LM75 code) to read temp

On AD7414 the first value upon bootup is not read correctly.
This is most likely because of the 800ms update time of the
temp register in normal update mode. To get current values
each time we issue the "dtt" command including upon powerup
we switch into one-short mode.

This patch fixes the problem on AD7414 equipped boards (Sequoia,
Canyonlands etc), that temp value printed in the bootup log was
incorrect.

Signed-off-by: Stefan Roese <sr@denx.de>