powerpc: Remove device_type = "rtc" properties in .dts files

We don't want to encourage the device_type usage.  It isn't used in
the code, so we can simply remove it from the dts files.

Suggested-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>