ARM: i.MX system: Add a reset fallback if base address of watchdog is not set

This patch adds a reset fallback if base address of watchdog is not set.
This is intended for a targets not compatible with imx-21 watchdog,
i.MX1 for example.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>