Add support for lantiq mips grx500 arch files

arch/mips/lantiq/grx500/irq.c

arch/mips/lantiq/grx500/prom.c

arch/mips/lantiq/grx500/prom.h

arch/mips/lantiq/grx500/time.c

arch/mips/lantiq/grx500_bootcore/Makefile

arch/mips/lantiq/grx500_bootcore/clk.c

arch/mips/lantiq/grx500_bootcore/clk.h

arch/mips/lantiq/grx500_bootcore/early_printk.c

arch/mips/lantiq/grx500_bootcore/interaptiv_reset.c

arch/mips/lantiq/grx500_bootcore/irq.c

arch/mips/lantiq/grx500_bootcore/ngi.c

arch/mips/lantiq/grx500_bootcore/ngi.h

arch/mips/lantiq/grx500_bootcore/ngi_map.h

arch/mips/lantiq/grx500_bootcore/ngi_reg.h

arch/mips/lantiq/grx500_bootcore/prom.c

arch/mips/lantiq/grx500_bootcore/prom.h

arch/mips/lantiq/grx500_bootcore/sysctrl.c

arch/mips/lantiq/grx500_bootcore/timer.c