ASoC: codecs: Kconfig: Show knob, and depend on MIPS || COMPILE_TEST

Show the knob to enable or disable the jz4740-codec driver, add a
proper description, and add a dependency on MIPS || COMPILE_TEST, as
this driver is only useful on MIPS.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Mark Brown <broonie@kernel.org>