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>
diff --git a/cpu/arm926ejs/versatile/timer.c b/cpu/arm926ejs/versatile/timer.c
index 50c1335..563db36 100755
--- a/cpu/arm926ejs/versatile/timer.c
+++ b/cpu/arm926ejs/versatile/timer.c
@@ -109,7 +109,7 @@
 }
 
 /* delay x useconds AND perserve advance timstamp value */
-void udelay (unsigned long usec)
+void __udelay (unsigned long usec)
 {
 	ulong tmo, tmp;