hwmon: Provide dtt_init()

Our boards rely on dtt for initialization of fan hardware.
dtt_init() was implemented to be called form board specific code.

Signed-off-by: Dirk Eibach <eibach@gdsys.de>
Acked-by: Heiko Schocher <hs@denx.de>
Tested-by: Heiko Schocher <hs@denx.de>