ARM: dts: i.MX27: Add label to CPU node

Add a label to i.MX27 CPU node. This change allows the reuse this node
in the upper levels of the DTS.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>