target/riscv: fix a typo with interrupt names

Interrupt names have been swapped in 205377f8 and do not follow
IRQ_*_EXT definition order.

Signed-off-by: Emmanuel Blot <emmanuel.blot@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210421133236.11323-1-emmanuel.blot@sifive.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>