ASoC: Allow SND_SOC_WM8978 to be selected manually

When using a DT-based multi-platform kernel, there's not always Kconfig
logic that selects the right codec driver.
Allow the user to manually select WM8978.

This is needed for Armadillo 800 EVA using a generic r8a7740 multi-platform
kernel.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Mark Brown <broonie@linaro.org>