Add customDecoder interface to rocket #819

Job	Run time
riscv-arch-test	0s
jtag-dtm-test (freechips.rocketchip.system.WithJtagDTMSystem_freechips.rocketchip.system.DefaultC...	21m 27s
emulator (DefaultSmallConfig)	12m 53s
jtag-dtm-test (freechips.rocketchip.system.WithJtagDTMSystem_freechips.rocketchip.system.DefaultR...	20m 10s
riscv-tests (DefaultConfig)	19m 50s
emulator (DualBankConfig)	13m 53s
jtag-dtm-test (freechips.rocketchip.system.WithJtagDTMSystem_freechips.rocketchip.system.WithDebu...	18m 31s
riscv-tests (DefaultBufferlessConfig)	19m 46s
jtag-dtm-test (freechips.rocketchip.system.WithJtagDTMSystem_freechips.rocketchip.system.WithDebu...	17m 7s
emulator (DualChannelConfig)	14m 14s
riscv-tests (DefaultRV32Config)	19m 47s
emulator (DualChannelDualBankConfig)	13m 26s
riscv-tests (TinyConfig)	12m 52s
riscv-tests (DefaultFP16Config)	13m 54s
emulator (RoccExampleConfig)	13m 39s
emulator (Edge128BitConfig)	14m 8s
emulator (Edge32BitConfig)	12m 56s
emulator (QuadChannelBenchmarkConfig)	13m 53s
emulator (EightChannelConfig)	13m 53s
emulator (DualCoreConfig)	14m 7s
emulator (MemPortOnlyConfig)	13m 4s
emulator (MMIOPortOnlyConfig)	12m 43s
emulator (CloneTileConfig)	14m 46s
emulator (HypervisorConfig)	13m 6s
	5h 54m 5s

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