RISC-V: Support scheduling for sifive p400 series

Add sifive p400 series scheduler module. For more information
see https://www.sifive.com/cores/performance-p450-470.

gcc/ChangeLog:

	* config/riscv/riscv.md: Include sifive-p400.md.
	* config/riscv/sifive-p400.md: New file.
	* config/riscv/riscv-cores.def (RISCV_TUNE): Add parameter.
	* config/riscv/riscv-opts.h (enum riscv_microarchitecture_type):
	Add sifive_p400.
	* config/riscv/riscv.cc (sifive_p400_tune_info): New.
	* config/riscv/riscv.h (TARGET_SFB_ALU): Update.
	* doc/invoke.texi (RISC-V Options): Add sifive-p400-series