spi: stm32-qspi: Add pm_runtime support

By default, STM32_AUTOSUSPEND_DELAY is set to -1 which has for
effect to prevent runtime suspends.
Runtime suspends can be activated by setting autosuspend_delay_ms using
sysfs entry :
echo {delay_in_ms} > /sys/devices/platform/soc/58003000.spi/power/autosusp
end_delay_ms)

Signed-off-by: Christophe Kerello <christophe.kerello@st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Link: https://lore.kernel.org/r/20200417121241.6473-1-patrice.chotard@st.com
Signed-off-by: Mark Brown <broonie@kernel.org>