timer: mpc83xx_timer: fix build with CONFIG_{HW_, }WATCHDOG

The code, which is likely copied from arch/powerpc/lib/interrupts.c,
lacks a fallback definition of CONFIG_SYS_WATCHDOG_FREQ and refers to
a non-existing timestamp variable - obviously priv->timestamp is
meant.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>