mtd: st_spi_fsm: Prepare read/write sequences according to configuration

Firstly we search for our preference read/write configuration based on a
given chip's capabilities. Then we actually set up the message sequence
accordingly.

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>