tcl/target/ti_k3: Add a gdb-attach event hook for m3 and m4

Add gdb-attach event to call the "up" function of m3 and m4 allowing for
more seamless integration with gdb for end users. We still retain _up
functions for non-gdb functionality.

NOTE: we add a halt 1000 to retain the default gdb-attach hook behavior

Suggested-by: Antonio Borneo <borneo.antonio@gmail.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Change-Id: I2e51fdbd8756f156551e589c748c3a338afa655c
Reviewed-on: https://review.openocd.org/c/openocd/+/6615
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>