doc: Updated RISC-V memory-related cmds in documentation

- "riscv set_prefer_sba" - removed from the doc, superseded
  by the latter
- "riscv set_mem_access" - new command, new entry added
  into the doc

This change only addresses the documentation. The corresponding
implementation of the RISC-V commands is already merged.

Change-Id: I3c07672cde94324407cf667504dba5402f63a543
Signed-off-by: Jan Matyas <matyas@codasip.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6743


Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Tested-by: jenkins