RISC-V: KVM: Support firmware events

SBI PMU extension defines a set of firmware events which can provide
useful information to guests about the number of SBI calls. As
hypervisor implements the SBI PMU extension, these firmware events
correspond to ecall invocations between VS->HS mode. All other firmware
events will always report zero if monitored as KVM doesn't implement them.

This patch adds all the infrastructure required to support firmware
events.

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