target/riscv: remove .instance_post_init

target/riscv: remove .instance_post_init

Unlike other uses of .instance_post_init, accel_cpu_instance_init()
*registers* properties, and therefore must be run before
device_post_init() which sets them to their values from -global.

In order to move all registration of properties to .instance_init,
call accel_cpu_instance_init() at the end of riscv_cpu_init().

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