mtd: st_spi_fsm: Use device size to determine address width

Take some known parameters, namely size and number of sectors and use
them to determine weather a device can support 32bit addressing or not.
If it can, set the associated flash capability flag for latter use.

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>