lib: provide a platform specific tlb range flush threshold

Currently, the tlb range flush threshold is fixed and set to 4k for
all platforms. However, it should be platform specific as it completely
depends upon how platform actually implements sfence instruction.

Define a platform feature that allows every individual platform to set
different values. If a platform doesn't define it, just use a page size as
the threshold.

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>