target: rewrite command 'arp_reset' as COMMAND_HANDLER

While there, add the missing .usage field and move in target.c the
enum nvp_assert.

Change-Id: Ia4f2f962887b5a35faeaa4eae128fa2865569b24
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7559
Tested-by: jenkins

src/target/target.h

@@ -57,11 +57,6 @@ enum target_state {
 	TARGET_DEBUG_RUNNING = 4,
 };
 
-enum nvp_assert {
-	NVP_DEASSERT,
-	NVP_ASSERT,
-};
-
 enum target_reset_mode {
 	RESET_UNKNOWN = 0,
 	RESET_RUN = 1,		/* reset and let target run */