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>