target/riscv: Fix read and write accesses to vsip and vsie

target/riscv: Fix read and write accesses to vsip and vsie

The previous implementation was broken in many ways:
 - Used mideleg instead of hideleg to mask accesses
 - Used MIP_VSSIP instead of VS_MODE_INTERRUPTS to mask writes to vsie
 - Did not shift between S bits and VS bits (VSEIP <-> SEIP, ...)

Signed-off-by: Georg Kotheimer <georg.kotheimer@kernkonzept.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210311094738.1376795-1-georg.kotheimer@kernkonzept.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>