tcl: Replace 'gdb_' prefix with 'gdb' command group

Change-Id: I0490b4c112c1a922bf77a4b37df2a630a8f6cea1
Signed-off-by: Marc Schink <dev@zapb.de>
Reviewed-on: https://review.openocd.org/c/openocd/+/8337
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Tested-by: jenkins

tcl/target/u8500.cfg

@@ -144,7 +144,7 @@ proc enable_apetap {} {
 
 tcl_port 5555
 telnet_port 4444
-gdb_port 3333
+gdb port 3333
 
 if { [info exists CHIPNAME] } {
 global _CHIPNAME
@@ -319,7 +319,7 @@ global _MAXSPEED
 adapter speed $_MAXSPEED
 
 
-gdb_breakpoint_override hard
+gdb breakpoint_override hard
 set mem inaccessible-by-default-off
 
 jtag_ntrst_delay 100