powerpc: Don't do generic calibrate_delay()

Currently we are wasting time calling the generic calibrate_delay()
function. We don't need it since our implementation of __delay() is
based on the CPU timebase. So instead, we use our own small
implementation that initializes loops_per_jiffy to something sensible
to make the few users like spinlock debug be happy

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>