RISC-V: KVM: Add support for SBI STA registers

KVM userspace needs to be able to save and restore the steal-time
shared memory address. Provide the address through the get/set-one-reg
interface with two ulong-sized SBI STA extension registers (lo and hi).
64-bit KVM userspace must not set the hi register to anything other
than zero and is allowed to completely neglect saving/restoring it.

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