cp/cmp: Add WATCHDOG_RESET in copy and compare loop

On some boards with a very short watchdog timeout, the "cp" and
"cmp" commands may reset the board. This patch adds some
watchdog resets inside the loops. Otherwise for example the lwmon5
board will reset while doing something like this:

=> cp.b fc000000 1000000 100000

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