[POWERPC] bootwrapper: Set timebase_period_ns from dt_fixup_cpu_clocks

This lets udelay() work properly on platforms which use dt_fixup_cpu_clocks.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>