gdb_server: print the target associated to the gdb port

gdb_server: print the target associated to the gdb port

While running OpenOCD on multi-target SoC, it's not immediate to
detect which target is associated to each GDB port. The log only
reports:
	Info : Listening on port 3333 for gdb connections
and a verbose debug log is required to get such info.

Promote to LOG_INFO() the existing debug message that already
reports the association, obtaining for each port:
	Info : starting gdb server for stm32mp15x.cpu0 on 3333
	Info : Listening on port 3333 for gdb connections

Change-Id: I1bd75655a3449222c959e6e82f5e0f8f5acd908a
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5525
Tested-by: jenkins
Reviewed-by: Jan Matyas <matyas@codasip.com>
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>