target/riscv: add more RISCVCPUDef fields

target/riscv: add more RISCVCPUDef fields

Allow using RISCVCPUDef to replicate all the logic of custom .instance_init
functions.  To simulate inheritance, merge the child's RISCVCPUDef with
the parent and then finally move it to the CPUState at the end of
TYPE_RISCV_CPU's own instance_init function.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>