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: d5be89a8 ("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>