dt-bindings: watchdog: add stm32mp1 support

This patch adds support of stm32mp1.
stm32mp1 requires 2 clocks lsi and pclk.

Signed-off-by: Ludovic Barre <ludovic.barre@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Guenter Roeck <Linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>