ARM: STi: DT: STiH407: Add Device Tree node for the LPC

On current ST platforms the LPC controls a number of functions.  This
patch enables support for the LPC Watchdog and LPC RTC devices on LPC1
and LPC2 respectively.

Signed-off-by: David Paris <david.paris@st.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Maxime Coquelin <maxime.coquelin@st.com>