mtd: st_spi_fsm: Initialise and configure the FSM for normal working conditions

This patch uses default values to initialise a connected flash chip. This
includes; a device soft reset, setting of a safe working frequency, a
switch into Fast Sequencing Mode, configuring of timing data and a purge
of the FIFO.

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>