riscv: Fix type of SiFive[EU]SocState, member parent_obj

Device "riscv.sifive.e.soc" is a direct subtype of TYPE_DEVICE, but
its instance struct SiFiveESoCState's member @parent_obj is
SysBusDevice instead of DeviceState.  Correct that.

Same for "riscv.sifive.u.soc"'s instance struct SiFiveUSoCState.

Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Alistair Francis <Alistair.Francis@wdc.com>
Cc: Sagar Karandikar <sagark@eecs.berkeley.edu>
Cc: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>
Cc: qemu-riscv@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20200609122339.937862-21-armbru@redhat.com>