arm64: Use _armreg.h in cpu.h

We only needed armreg.h for the *_SPECIALREG macros. Use the new
_armreg.h directly.

Reviewed by:	emaste
Sponsored by:	Arm Ltd
Differential Revision:	https://reviews.freebsd.org/D53325

sys/arm64/include/_armreg.h

@@ -28,6 +28,7 @@
  */
 
 #if !defined(_MACHINE_ARMREG_H_) && \
+    !defined(_MACHINE_CPU_H_) && \
     !defined(_MACHINE_HYPERVISOR_H_)
 #error Do not include this file directly
 #endif

sys/arm64/include/cpu.h

@@ -43,10 +43,10 @@
 #define	_MACHINE_CPU_H_
 
 #if !defined(__ASSEMBLER__)
+#include <machine/_armreg.h>
 #include <machine/atomic.h>
 #include <machine/frame.h>
 #endif
-#include <machine/armreg.h>
 
 #define	TRAPF_PC(tfp)		((tfp)->tf_elr)
 #define	TRAPF_USERMODE(tfp)	(((tfp)->tf_spsr & PSR_M_MASK) == PSR_M_EL0t)