mtd: st_spi_fsm: Fetch boot-device from mode pins

It's important for us to determine which device was used to boot from in
order to make some correct decisions surrounding Power Management. On
each of the platforms which support the FSM this is communicated via
a set of mode pins held in the system configuration area. This patch
determine the boot device and stores the result.

Acked-by Angus Clark <angus.clark@st.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>