powerpc: Add a vga alias node for P1022

In u-boot, when set the video as console, the name 'vga' is used
as a general name for the video device, during the fdt_fixup_stdout
process, the 'vga' name is used to search in the dtb to setup the
'linux,stdout-path' node. Though the P1022 DIU is not VGA-compatible
device, to meet the 'vga' name used in u-boot, the vga alias node is
added for P1022 in this patch. At the same time, a display alias is
also added so that no other components grow dependencies on the vga
alias node.

Signed-off-by: Jason Jin <Jason.Jin@freescale.com>
Signed-off-by: Wang Dongsheng <dongsheng.wang@freescale.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>