serial: 8250: Add hardware dependency to RT288X option

Kconfig option SERIAL_8250_RT288X seems to be only relevant on MIPS
platforms, so do not present it on other architectures, unless
build-testing.
Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Mans Rullgard <mans@mansr.com>
Cc: Jiri Slaby <jslaby@suse.com>
Acked-by: John Crispin <blogic@openwrt.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/Kconfig

@@ -319,6 +319,7 @@ config SERIAL_8250_EM
 config SERIAL_8250_RT288X
 	bool "Ralink RT288x/RT305x/RT3662/RT3883 serial port support"
 	depends on SERIAL_8250
+	depends on MIPS || COMPILE_TEST
 	default y if MIPS_ALCHEMY || SOC_RT288X || SOC_RT305X || SOC_RT3883 || SOC_MT7620
 	help
 	  Selecting this option will add support for the alternate register