pwm: jz4740: Depend on MACH_INGENIC instead of MIPS

The MACH_INGENIC Kconfig option will be selected when building a kernel
targeting Ingenic SoCs, but also when compiling a generic MIPS kernel
that happens to support Ingenic SoCs.

Therefore, if MACH_INGENIC is not set, we know that we're not even
trying to build a generic kernel that supports these SoCs, and we can
hide the options to compile the SoC-specific drivers.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>