[POWERPC] 4xx: Create common ppc4xx_reset_system() in ppc4xx_soc.c

This patch creates a common system reset routine for all 40x and 44x
systems. Previously only a 44x routine existed. But since this system
reset via the debug control register is common for 40x and 44x let's
share this code for all those platforms in ppc4xx_soc.c.

This patch also enables CONFIG_4xx_SOC for all 40x and 44x platforms.

Tested on Kilauea (405EX) and Canyonlands (440EX).

Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>