KVM: arm64: Move hyp_symbol_addr() to kvm_asm.h

Recent refactoring of the arm64 code make it awkward to have
hyp_symbol_addr() in kvm_mmu.h. Instead, move it next to its
main user, which is __hyp_this_cpu_ptr().

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