powerpc/85xx: Rename P1021RDB-PC device trees to be consistent

The board is really P1021RDB-PC, so rename from p1021rdb.* to p1021rdb-pc.*

Signed-off-by: Xu Jiucheng <Jiucheng.Xu@freescale.com>
Signed-off-by: Matthew McClintock <msm@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>