spi: sprd: Add the support of restarting the system

On Spreadtrum platform, we use one PMIC watchdog to reset the whole system
with loading one suitable timeout value (usually 50ms) for the watchdog.

In theory, we should implement the restart function in drivers/power/reset
subsystem to access the PMIC watchdog with regmap. When restart the system,
other cores will be stopped by IPI, but if other cores were accessing PMIC
with holding the regmap mutex lock, that will cause dead-lock issue if we
try to access the PMIC watchdog with regmap to restart the whole system.
Thus we can implement the restart function in ADI driver to avoid this
issue.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>