i2c: at91: ensure state is restored after suspending

When going to suspend, the I2C registers may be lost because the power to
VDDcore is cut. Restore them when resuming.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>