MIPS: Remove not used 8250-platform.c

When CONFIG_HAVE_STD_PC_SERIAL_PORT is set, there exists build errors
of 8250-platform.c due to linux/module.h is not included.

CONFIG_HAVE_STD_PC_SERIAL_PORT is not used in arch/mips for many years,
8250-platform.c is also not built and used, so it is not necessary to
fix the build errors, just remove the not used file 8250-platform.c and
the related code in Kconfig and Makefile.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>