riscv: Implement Guest/VM VCPU arch functions

This patch implements kvm_cpu__<xyz> Guest/VM VCPU arch functions.

These functions mostly deal with:
1. VCPU allocation and initialization
2. VCPU reset
3. VCPU show/dump code
4. VCPU show/dump registers

We also save RISC-V ISA, XLEN, and TIMEBASE frequency for each VCPU
so that it can be later used for generating Guest/VM FDT.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Anup Patel <anup.patel@wdc.com>
Link: https://lore.kernel.org/r/20211119124515.89439-5-anup.patel@wdc.com


Signed-off-by: Will Deacon <will@kernel.org>