riscv: defconfig: enable hugetlbfs option

Enabling hugetlbfs in the defconfigs to allow tuning KVM
guest performance using huge pages.

Signed-off-by: Mayuresh Chitale <mchitale@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>