Some boards (e.g. lwmon5) may use rather small watchdog intervals, so
causing it to reboot the board if U-Boot does a long busy-wait with
udelay(). Thus, for these boards we have to restart WD more
frequently.

This patch splits the busy-wait udelay() into smaller, predefined,
intervals, so that the watchdog timer may be resetted with the
configurable (CONFIG_WD_PERIOD) interval.

Signed-off-by: Yuri Tikhonov <yur@emcraft.com>