tcl/target/ti_k3: Add a gdb-attach event hook for r5 and simplify startup function

tcl/target/ti_k3: Add a gdb-attach event hook for r5 and simplify startup function

Since we can detect the type of target as well, make the attach
function name generic for the follow on cleanup patch on armv8 to use
as well.

Lets introduce gdb-attach event in a much cleaner fashion. We can
introduce a simpler r5_up function since we now have more descriptive
core names making the individual descriptive procs redundant.

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

Signed-off-by: Nishanth Menon <nm@ti.com>
Change-Id: I31506bb2b86e63638082640eb72aa7c4c9575e93
Reviewed-on: https://review.openocd.org/c/openocd/+/6617
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>