mtd: st_spi_fsm: Add the ability to read from a Serial Flash device

When a read is issued by userspace the MTD framework calls back into
the driver to conduct the actual command issue and data extraction.
Here we provide the routines which do exactly that.

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>