lib: Change tlb range flush threshold to 4k page instead of 1G

In RISC-V, tlb flush happens at a page granularity. That's why OpenSBI
also have a tlb range flush limit which decides the which tlb flush
requests should be upgraded to full flush to avoid long delays.
Currently, this is set to 1G which would result in a many sfence.vma
execution in a tight loop for a large range.

Change the threshold to 4k to speed things up.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>