ASoC: atmel-classd: sync regcache when resuming

The PM functions used in this driver are the ones defined in
sounc/soc/soc-core.c.

When suspending (using snd_soc_suspend), the regcache is marked dirty
but is never synced on resume.

Sync regcache on resume of Atmel ClassD device.

Signed-off-by: Quentin Schulz <quentin.schulz@free-electrons.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Mark Brown <broonie@kernel.org>