target/riscv: rvv-1.0: relax RV_VLEN_MAX to 1024-bits

Signed-off-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20211210075704.23951-66-frank.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>