Introduce driver for STM32 IWDG peripheral (Independent Watchdog).
It is configured according to device tree content and should be enabled
from there.
The watchdog is not started by default. It can be started after an HW
reset if the dedicated OTP is fused.

The watchdog also needs to be frozen if a debugger is attached.
This is done by configuring the correct bits in DBGMCU.
This configuration is allowed by checking BSEC properties.

An increase of BL2 size is also required when adding this new code.

Change-Id: Ide7535d717885ce2f9c387cf17afd8b5607f3e7f
Signed-off-by: Yann Gautier <yann.gautier@st.com>
Signed-off-by: Lionel Debieve <lionel.debieve@st.com>
Signed-off-by: Nicolas Le Bayon <nicolas.le.bayon@st.com>