target/mips32: add dsp access support

Add access to dsp registers and a command for dsp related operations.
Checkpatch-ignore: MACRO_ARG_REUSE

Change-Id: I30aec0b9e4984896965edb1663f74216ad41101e
Signed-off-by: Walter Ji <walter.ji@oss.cipunited.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7867
Tested-by: jenkins
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>

doc/openocd.texi

@@ -11070,6 +11070,11 @@ EJTAG Register Specification could be found in MIPS Document MD00047F, for
 core specific EJTAG Register definition, please check Core Specific SUM manual.
 @end deffn
 
+@deffn {Command} {mips32 dsp} [[register_name] [value]]
+Displays all DSP registers' contents or get/set value by register name. Will display
+an error if current CPU does not support DSP.
+@end deffn
+
 @section RISC-V Architecture
 
 @uref{http://riscv.org/, RISC-V} is a free and open ISA. OpenOCD supports JTAG