[POWERPC] Replace use of GET_64BIT(prop, i) with of_read_number().

Remove uses of hack GET_64BIT() property macro and use
the more general of_read_number() function from prom.h
as suggested by Milton.

Signed-off-by: Jon Loeliger <jdl@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>