arch_helpers: use u_register_t for register read/write

u_register_t is preferred rather than uint64_t.  This is more
consistent with the aarch32 implementation.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

include/lib/aarch64/arch_helpers.h

@@ -18,15 +18,15 @@
  *********************************************************************/
 
 #define _DEFINE_SYSREG_READ_FUNC(_name, _reg_name)		\
-static inline uint64_t read_ ## _name(void)			\
+static inline u_register_t read_ ## _name(void)			\
 {								\
-	uint64_t v;						\
+	u_register_t v;						\
 	__asm__ volatile ("mrs %0, " #_reg_name : "=r" (v));	\
 	return v;						\
 }
 
 #define _DEFINE_SYSREG_WRITE_FUNC(_name, _reg_name)			\
-static inline void write_ ## _name(uint64_t v)				\
+static inline void write_ ## _name(u_register_t v)			\
 {									\
 	__asm__ volatile ("msr " #_reg_name ", %0" : : "r" (v));	\
 }