tcl/target/ti_k3: Rename m4 target as general purpose mcu

The MCU is present on few of the SoCs and is meant as General Purpose
(GP) MCU of the system. Lets rename it to make clear what we are
debugging - esp when multiple MCUs are present in the system.

Signed-off-by: Nishanth Menon <nm@ti.com>
Change-Id: I16132d321daf6e9b1d893fe6f92026d5aa9eb152
Reviewed-on: https://review.openocd.org/c/openocd/+/6619
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>