RISC-V: Define get_cycles64() regardless of M-mode

The timer driver uses get_cycles64() unconditionally to obtain the current
time.  A recent refactoring lost the common definition for some configs, which
is now the only one we need.

Fixes: d5be89a8d1 ("RISC-V: Resurrect the MMIO timer implementation for M-mode systems")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/include/asm/timex.h

@@ -60,6 +60,8 @@ static inline u32 get_cycles_hi(void)
 }
 #define get_cycles_hi get_cycles_hi
 
+#endif /* !CONFIG_RISCV_M_MODE */
+
 #ifdef CONFIG_64BIT
 static inline u64 get_cycles64(void)
 {
@@ -79,8 +81,6 @@ static inline u64 get_cycles64(void)
 }
 #endif /* CONFIG_64BIT */
 
-#endif /* !CONFIG_RISCV_M_MODE */
-
 #define ARCH_HAS_READ_CURRENT_TIMER
 static inline int read_current_timer(unsigned long *timer_val)
 {