powerpc/kernel: Use of_property_read_u32() in rtas_initialize()

This uses of_property_read_u32() in rtas_initialize() so that we
needn't explicitly care the CPU's endian.

Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>