RISC-V: KVM: Add IMSIC related defines

We add IMSIC related defines in a separate header so that different
parts of KVM code can share it. Once AIA drivers are merged will
have a common IMSIC header shared by both KVM and IRQCHIP driver.

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