atmel_flexcom: Support resuming after a chip reset

The controller used by a flexcom module is configured at boot, and left
alone after this. In the suspend mode called "backup with self-refresh"
available on SAMA5D2, the chip will resume with most of its registers
reset. In this case, we need to restore the state of the flexcom driver
on resume.

Signed-off-by: Romain Izard <romain.izard.pro@gmail.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Tested-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>