arm: hyp vectors installation

arm: hyp vectors installation

The EL2 installation is done in two times:
- First, an HVC is issued to jump into the kernel stub and install the
  bootstrap vectors.
- Then, a second HVC allows the setup code to switch to physical address
  space.
Execution continues at EL2. Once the whole initialisation is done, the
final vectors are installed, and arch_cpu_activate_vmm will do an ERET
to jump back to the kernel, which is now a guest.

Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>