watchdog: da9062: make restart handler atomic safe

The restart handler is executed during the shutdown phase which is
atomic/irq-less. The i2c framework supports atomic transfers since
commit 63b96983 ("i2c: core: introduce callbacks for atomic
transfers") to address this use case. Using regmap within an atomic
context is allowed only if the regmap type is MMIO and the cache type
'flat' or no cache is used. Using the i2c_smbus_write_byte_data()
function can be done without additional tests because:
 1) the DA9062 is an i2c-only device and
 2) the i2c framework emulates the smbus protocol if the host adapter
    does not support smbus_xfer by using the master_xfer.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Stefan Lengfeld <contact@stefanchrist.eu>
Tested-by: Stefan Lengfeld <contact@stefanchrist.eu>
Reviewed-by: Adam Thomson <Adam.Thomson.Opensource@diasemi.com>
Link: https://lore.kernel.org/r/20200115162307.7336-1-m.felsch@pengutronix.de


Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>