i2c: owl: Add support for atomic transfers

Atomic transfers are required to properly power off a machine through
an I2C controlled PMIC, such as the Actions Semi ATC260x series.

System shutdown may happen with interrupts being disabled and, as a
consequence, the kernel may hang if the driver does not support atomic
transfers.

This functionality is essentially implemented by polling the FIFO
Status register until either Command Execute Completed or NACK Error
bits are set.

Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@gmail.com>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
[wsa: rebased to current tree]
Signed-off-by: Wolfram Sang <wsa@kernel.org>