mISDN: Add ISDN sample clock API to mISDN core

Add ISDN sample clock API to mISDN core (new file clock.c)
hfcmulti and mISDNdsp use clock API.

Signed-off-by: Andreas Eversberg <andreas@eversberg.eu>
Signed-off-by: Karsten Keil <kkeil@suse.de>