arm64: add hypervisor stub

If booted in EL2, install an dummy hypervisor whose only purpose
is to be replaced by a full fledged one.

A minimal API allows to:
- obtain the current HYP vectors (__hyp_get_vectors)
- set new HYP vectors (__hyp_set_vectors)

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/virt.h

@@ -33,6 +33,9 @@
  */
 extern u32 __boot_cpu_mode[2];
 
+void __hyp_set_vectors(phys_addr_t phys_vector_base);
+phys_addr_t __hyp_get_vectors(void);
+
 /* Reports the availability of HYP mode */
 static inline bool is_hyp_mode_available(void)
 {