[MIPS] Put an end to <asm/serial.h>'s long and annyoing existence

arch/mips/Kconfig

arch/mips/ddb5xxx/ddb5477/Makefile

arch/mips/ddb5xxx/ddb5477/ddb5477-platform.c

arch/mips/gt64120/momenco_ocelot/Makefile

arch/mips/gt64120/momenco_ocelot/ocelot-platform.c

arch/mips/jazz/Makefile

arch/mips/jazz/jazz-platform.c

arch/mips/kernel/8250-platform.c

arch/mips/kernel/Makefile

arch/mips/mips-boards/malta/Makefile

arch/mips/mips-boards/malta/malta_platform.c

arch/mips/momentum/ocelot_3/platform.c

arch/mips/sgi-ip32/ip32-platform.c

arch/mips/sgi-ip32/ip32-setup.c

include/asm-mips/serial.h