The wm8940 driver is using a regmap cache sync to restore the
configuration of the chip when switching from OFF to STANDBY, but does
not actually define a register cache which means that the restore is
never going to work and we trigger asserts in regmap.  Fix this by
enabling caching.

Based on commit d3030d11 ("ASoC: ak4642: Enable cache usage to
fix crashes on resume") by Mark Brown <broonie@kernel.org>.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>