Create riscv repeat_read command (#510)

* WIP, apply stash with conflicts.

Change-Id: Ia794bde419aa29161c68898d20e30527e69f5a31

* Fix conflict resolution problems.

Change-Id: I4cedc348cf613f98cc5a36886f37c568ca644238

* Add repeat_read command.

Only implemented for sba v1 right now, and poorly tested at that.

Change-Id: I1d9ff63e1dea14b3f6a9f8ba4dad53668bf8038b

* Hide bogus address in repeat_read

Change-Id: Ib66c1fa60df9c7fc7cc87880b0fddc52825b48aa

* WIP make repeat read work with progbuf.

Change-Id: I555f8b880c8bf0d1ed0f3f90c7987a5b516a7a79

* WIP

Change-Id: Ic567cea68355ae907e94bd25185a2c9be6fd798d

* Fix error handling when increment is non-zero.

Change-Id: I5a2f3f2ee948fd4e12c0443a542e85b7b5c5791a

* Correctly(?) handle failures when increment is 0.

I'm not 100% convinced that this ensures every read value shows up in
the output, but it ought to work.

Change-Id: I1af3e7174cf9d5e6f293456fb5ead629e17faaaa

* Don't crash when asked to read no data.

Change-Id: I4061b5c720a43a4f828384ab9eacc89557adfa05

* Remove unnecessary comment.

Change-Id: I1be3d699b86299339b3a830ca1ef13c9f5b9fe0f

* Document `riscv repeat_read`.

Change-Id: I4a0f071f38784b2de034f8c1b0ce75d6d2d326b2

doc/openocd.texi

@@ -9659,6 +9659,12 @@ indicates the first custom register, whose abstract command number is 0xc000.
 This command must be executed before `init`.
 @end deffn
 
+@deffn Command {riscv repeat_read} count address [size=4]
+Quickly read count words of the given size from address. This can be useful
+to read out a buffer that's memory-mapped to be accessed through a single
+address, or to sample a changing value in a memory-mapped device.
+@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).