riscv: Add Svpbmt extension support

The Svpbmt extension allows PTE based memory attributes in page tables.
This extension also allows Guest/VM to use PTE based memory attributes
in VS-stage page tables so let us add it Guest/VM ISA string when KVM
RISC-V supports it.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Link: https://lore.kernel.org/r/20220815101325.477694-4-apatel@ventanamicro.com


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