target/cortex_a: add hypervisor mode

target/cortex_a: add hypervisor mode

Hypervisor mode is present only if the optional virtualization
extensions are available. Moreover, virtualization extensions
require that also security extensions are implemented.

Add the required infrastructure for the shadowed registers in
hypervisor mode.
Make monitor shadowed registers visible in hypervisor mode too.
Make hypervisor shadowed registers visible in hypervisor mode
only.
Check during cortex_a examine if virtualization extensions are
present and then conditionally enable the visibility of both
hypervisor and monitor modes shadowed registers.

Change-Id: I81dbb1ee8baf4c9f1a2226b77c10c8a2a7b34871
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5261
Tested-by: jenkins