hw/riscv: Fix OT IBEX reset vector

hw/riscv: Fix OT IBEX reset vector

The IBEX documentation [1] specifies the reset vector to be "the most
significant 3 bytes of the boot address and the reset value (0x80) as
the least significant byte".

[1] https://github.com/lowRISC/ibex/blob/master/doc/03_reference/exception_interrupts.rst

Signed-off-by: Alexander Wagner <alexander.wagner@ulal.de>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210420080008.119798-1-alexander.wagner@ulal.de
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>