RISC-V: KVM: Factor-out g-stage page table management

The upcoming nested virtualization can share g-stage page table
management with the current host g-stage implementation hence
factor-out g-stage page table management as separate sources
and also use "kvm_riscv_mmu_" prefix for host g-stage functions.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Tested-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Nutty Liu <liujingqi@lanxincomputing.com>
Link: https://lore.kernel.org/r/20250618113532.471448-12-apatel@ventanamicro.com
Signed-off-by: Anup Patel <anup@brainfault.org>

arch/riscv/kvm/Makefile

@@ -14,6 +14,7 @@ kvm-y += aia.o
 kvm-y += aia_aplic.o
 kvm-y += aia_device.o
 kvm-y += aia_imsic.o
+kvm-y += gstage.o
 kvm-y += main.o
 kvm-y += mmu.o
 kvm-y += nacl.o