ASoC: cs42l51: make driver user-selectable

Since we are removing the Armada 370 DB audio machine driver to use
the 'simple-card' Device Tree binding, we can no longer select the
CS42L51 codec driver using a Kconfig 'select', and we instead need it
to be user-selectable. Therefore, this commit adds a prompt to make
the CS42L51 I2C codec driver user-selectable.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Brian Austin <brian.austin@cirrus.com>
Signed-off-by: Mark Brown <broonie@kernel.org>