target/riscv: cpu: Add a new 'resetvec' property

Currently the reset vector address is hard-coded in a RISC-V CPU's
instance_init() routine. In a real world we can have 2 exact same
CPUs except for the reset vector address, which is pretty common in
the RISC-V core IP licensing business.

Normally reset vector address is a configurable parameter. Let's
create a 64-bit property to store the reset vector address which
covers both 32-bit and 64-bit CPUs.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <1598924352-89526-2-git-send-email-bmeng.cn@gmail.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>