kdb: Rename struct defcmd_set to struct kdb_macro

Rename struct defcmd_set to struct kdb_macro as that sounds more
appropriate given its purpose.

Suggested-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20210712134620.276667-2-sumit.garg@linaro.org


Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>