Store current vcpu instead of cpu in tpidr_el2.

This allows us to avoid an extra indirection in the most common paths.

Change-Id: I2ea71ee1a56ee8b94f7f516465081e88e82d8539
diff --git a/inc/hf/cpu.h b/inc/hf/cpu.h
index 809c21e..1d3d640 100644
--- a/inc/hf/cpu.h
+++ b/inc/hf/cpu.h
@@ -45,6 +45,7 @@
 struct vcpu {
 	struct spinlock lock;
 	enum vcpu_state state;
+	struct cpu *cpu;
 	struct vm *vm;
 	struct vcpu *mailbox_next;
 	struct arch_regs regs;
@@ -55,8 +56,6 @@
 	/* CPU identifier. Doesn't have to be contiguous. */
 	size_t id;
 
-	struct vcpu *current;
-
 	/* Pointer to bottom of the stack. */
 	void *stack_bottom;