platform/mips: Adjust Kconfig to keep consistency

Drop the "if MIPS" and "endif" drivers/platform/Kconfig. Instead add
a "depends on MIPS" to "menuconfig MIPS_PLATFORM_DEVICES" in drivers/
platform/mips/Kconfig, like all the other subdirs are doing.

Suggested-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>