ASoC: rt5645: Make the Kconfig symbol user selectable

ASoC: rt5645: Make the Kconfig symbol user selectable

SND_SOC_RT5645 has no prompt, so it can only be turned on by a machine
driver that selects it. Every such driver is x86, MediaTek or Rockchip,
which leaves the codec unreachable on any other platform.

The part has a devicetree binding of its own,
Documentation/devicetree/bindings/sound/realtek,rt5645.yaml, and nothing
in the driver is machine specific, so a board that describes it with
simple-audio-card or audio-graph-card should be able to build it. Today
there is no configuration in which that is possible.

Give the symbol a prompt, matching SND_SOC_RT5640 immediately above it.
The machine drivers that select it are unaffected.

Signed-off-by: Rudi Heitbaum <rudi@heitbaum.com>
Link: https://patch.msgid.link/anCcciWmO6QOLTsl@5e001e58230e
Signed-off-by: Mark Brown <broonie@kernel.org>