riscv: cpu.c: don't use kernel-doc markers for comments

Repair kernel-doc build warnings caused by using "/**" kernel-doc
markers for comments that are not in kernel-doc format:

cpu.c:89: warning: cannot understand function prototype: 'struct riscv_isa_ext_data isa_ext_arr[] = '
cpu.c:114: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>