mtd: st_spi_fsm: Erase partly or as a whole a Serial Flash device

When an erase is requested by userspace the MTD framework calls back
into the driver to conduct the actual command issue. Here we provide the
routines which do exactly that. We can choose to either do an entire chip
erase or by sector.

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>