riscv: Fixup CONFIG_GENERIC_TIME_VSYSCALL

The patch fix commit: ad5d1122 ("riscv: use vDSO common flow to
reduce the latency of the time-related functions").

The GENERIC_TIME_VSYSCALL should be CONFIG_GENERIC_TIME_VSYSCALL
or vgettimeofday won't work.

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Reviewed-by: Pekka Enberg <penberg@kernel.org>
Fixes: ad5d1122 ("riscv: use vDSO common flow to reduce the latency of the time-related functions")
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>