target/aarch64: fix use of 'target->private_config'

target/aarch64: fix use of 'target->private_config'

The function adiv5_jim_configure() casts the void pointer
'target->private_config' to a struct adiv5_private_config pointer.
This is tricky in case of aarch64, where the private data are in a
struct aarch64_private_config that has as first element the struct
adiv5_private_config.

While the current solution is working fine, it's not clean and
requires special attention for any further code development.

Override 'target->private_config' to the correct pointer while
calling adiv5_jim_configure().

Change-Id: Ic2fc047dd1e57013943d96e6d5879a919d1eb7b3
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5847
Tested-by: jenkins
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>