clocksource: clint: Export clint_time_val for modules

clint_time_val will soon be used by the RISC-V implementation of
random_get_entropy(), which is a static inline function that may be used by
modules (at least CRYPTO_JITTERENTROPY=m).

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>