target/stm32h7x: Use AP2 to access DBGMCU when non HLA adapter is used

target/stm32h7x: Use AP2 to access DBGMCU when non HLA adapter is used

The STM32H7 has three access ports. The DBGMCU component is available
through AP0 at 0x5C001000 and through AP2 at 0xE00E1000. Using the
latter is preferable for early configuration because it works in all
power states and while SRST is asserted, whereas the former does not.

Change-Id: Iaf8f01d769efb6655040060a8e1e951e1f7e50ab
Signed-off-by: Christopher Head <chead@zaber.com>
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: http://openocd.zylin.com/4742
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>