powerpc/mpc52xx: add a4m072 board support

Add DTS file for a4m072 board and add its name to the list
of the supported boards.

Signed-off-by: Heiko Schocher <hs@denx.de>
cc: Grant Likely <grant.likely@secretlab.ca>
cc: devicetree-discuss@ozlabs.org
cc: Wolfgang Denk <wd@denx.de>
cc: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Anatolij Gustschin <agust@denx.de>