hw/riscv: Drop CONFIG_SIFIVE

hw/riscv: Drop CONFIG_SIFIVE

The name SIFIVE is too vague to convey the required component of
MSI_NONBROKEN. Let's drop the option, and select MSI_NONBROKEN in
each machine instead.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <1599129623-68957-12-git-send-email-bmeng.cn@gmail.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>