Generic udelay() with watchdog support

According to the PPC reference implementation the udelay() function is
responsible for resetting the watchdog timer as frequently as needed.
Most other architectures do not meet that requirement, so long-running
operations might result in a watchdog reset.

This patch adds a generic udelay() function which takes care of
resetting the watchdog before calling an architecture-specific
__udelay().

Signed-off-by: Ingo van Lil <inguin@gmx.de>