mfd: dt-bindings: Provide human readable defines for LPC mode choosing

ST's Low Power Controller can currently operate in two supported modes;
Watchdog and Real Time Clock.  These defines will aid engineers to easily
identify the selected mode.

Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Maxime Coquelin <maxime.coquelin@st.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>