gdb/riscv: add ability to decode dwarf CSR numbers

Extends riscv_dwarf_reg_to_regnum to add the ability to convert the
DWARF register numbers for CSRs into GDB's internal numbers.

gdb/ChangeLog:

	* riscv-tdep.c (riscv_dwarf_reg_to_regnum): Decode DWARF CSR
	numbers.
	* riscv-tdep.h (RISCV_DWARF_FIRST_CSR, RISCV_DWARF_LAST_CSR): New
	enum values.