Add command to expose custom registers (#293)

* Added `riscv expose_custom` command.

Seems to work for reading. I need to do some more testing for writes, as
well as minor cleanup.

Change-Id: I85d5d00897d5da4add4a6643b538be37d31a016f

* Conform to OpenOCD style.

Change-Id: I40a316f06f418d2b63d9e11aea03ef51da8d8faf

* Free all the memory allocated by register init.

Change-Id: I04e35ab54613f99708cee85e41fef989079adefc

* Properly document `riscv expose_custom`.

Change-Id: Id78a02b7a00c161df80f11b521a306e0cf3d7478

doc/openocd.texi

@@ -8998,6 +8998,14 @@ command can be used if OpenOCD gets this wrong, or a target implements custom
 CSRs.
 @end deffn
 
+@deffn Command {riscv expose_csrs} n0[-m0][,n1[-m1]]...
+The RISC-V Debug Specification allows targets to expose custom registers
+through abstract commands. (See Section 3.5.1.1 in that document.) This command
+configures a list of inclusive ranges of those registers to expose. Number 0
+indicates the first custom register, whose abstract command number is 0xc000.
+This command must be executed before `init`.
+@end deffn
+
 @deffn Command {riscv set_command_timeout_sec} [seconds]
 Set the wall-clock timeout (in seconds) for individual commands. The default
 should work fine for all but the slowest targets (eg. simulators).