KVM: arm64: nv: Handle CPACR_EL1 traps

Handle CPACR_EL1 accesses when running a VHE guest. In order to
limit the cost of the emulation, implement it ass a shallow exit.

In the other cases:

- this is a nVHE L1 which will write to memory, and we don't trap

- this is a L2 guest:

  * the L1 has CPTR_EL2.TCPAC==0, and the L2 has direct register
   access

  * the L1 has CPTR_EL2.TCPAC==1, and the L2 will trap, but the
    handling is defered to the general handling for forwarding

Signed-off-by: Marc Zyngier <maz@kernel.org>