KVM: arm64: Add kimg_hyp_va() helper

KVM/arm64 is so far unable to deal with function pointers, as the compiler
will generate the kernel's runtime VA, and not the linear mapping address,
meaning that kern_hyp_va() will give the wrong result.

We so far have been able to use PC-relative addressing, but that's not
always easy to use, and prevents the implementation of things like
the mapping of an index to a pointer.

To allow this, provide a new helper that computes the required
translation from the kernel image to the HYP VA space.

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