target/riscv: remove csr.h from kvm-cpu.c

Move riscv_new_csr_seed from csr.c to cpu.c since this function is
shared with KVM.  With that we can remove the csr.h from kvm-cpu.c.

Signed-off-by: Daniel Henrique Barboza <daniel.barboza@oss.qualcomm.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20260703180538.3346781-10-daniel.barboza@oss.qualcomm.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>