lib: Implement sfence.vma correctly.

Currently, OpenSBI doesn't distinguish between sfence.vma
and sfence.vm asid calls. Moreover, it ignores the page
ranges and just flush entire TLB everytime.

Fix the sfence implementation by keeping all the tlb flush
info in scratch area.

The relevant Linux kernel code was added by
https://patchwork.kernel.org/project/linux-riscv/list/?series=89695



However, this patch is backward compatible with older version kernel
that doesn't have the above patches as well.

Fixes #87
Signed-off-by: Atish Patra <atish.patra@wdc.com>