RISC-V: KVM: Move sbi related struct and functions to kvm_vcpu_sbi.h

Just like asm/kvm_vcpu_timer.h, we should have all sbi related struct
and functions in asm/kvm_vcpu_sbi.h.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Anup Patel <anup@brainfault.org>