i2c: stm32f7: Add atomic_xfer method to driver

Add an atomic_xfer method to the driver so that it behaves correctly
when controlling a PMIC that is responsible for device shutdown.

The atomic_xfer method added is similar to the one from the i2c-mv64xxx
driver. When running an atomic_xfer a bool flag in the driver data is
set, the interrupt is not unmasked on transfer start, and the IRQ
handler is manually invoked while waiting for pending transfers to
complete.

Signed-off-by: Sean Nyekjaer <sean@geanix.com>
Acked-by: Andi Shyti <andi.shyti@kernel.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>