stimecmp: perform menvcfg.STCE permission check when accessing vstimecmp in HS-mode

The spec requires menvcfg.STCE=1 on accessing stimecmp or vstimecmp in a
mode other than M-mode. The previous implementation does not check the
permission on accessing vstimecmp in HS-mode. This commit fixes the
issue by moveing the permission check from virtualized_stimecmp_csr_t to
stimecmp_csr_t, which implements the vstimecmp.