rvv: vstart register needs only lg2(VLEN) bits (#501)

As section 3.6 says:
  The vstart CSR is defined to have only enough writable bits to hold
  the largest element index (one less than the maximum VLMAX) or lg2(VLEN) bits.

Signed-off-by: Chih-Min Chao <chihmin.chao@sifive.com>