Per simple-bus.yaml, 'aipi' is not a valid node name.

Change it to 'bus' to fix the following dt-schema warning:

imx27-pdk.dtb: aipi@10020000: $nodename:0: 'aipi@10020000' does not match '^([a-z][a-z0-9\\-]+-bus|bus|localbus|soc|axi|ahb|apb)(@.+)?$'
	from schema $id: http://devicetree.org/schemas/simple-bus.yaml#Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>