mtd: st_spi_fsm: Supply all register address and bit logic defines

Here we provide the FSM's register addresses, register bit names/offsets
and some commands which will prove useful as we start bulk the FMS's
driver out with functionality.

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>