target/openrisc: Move VR, UPR, DMMCFGR, IMMCFGR to cpu init

These registers are read-only and implementation specific.
Initiailize VR for the first time; take the OR1200 values
from the verilog source.

Note that moving fields within CPUOpenRISCState does not
affect migration.

Reviewed-by: Stafford Horne <shorne@gmail.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>