mpc5200: a4m2k: Implement custom "dynamic" watchdog support

This patch adds a custom U-Boot command "wdogtoggle" which enables the
external hardware watchdog toggling via an GPIO pin on the a4m2k
board. After issuing this commands, the watchdog will be serviced
by U-Boot so that the user can use all U-Boot commands from the
prompt.

Signed-off-by: Stefan Roese <sr@denx.de>