platform: Add an platform ops to return platform specific tlb flush limit

If a platform requires to perform a tlb full flush, they should set
the tlb_range_flush_limit value to zero. However, corresponding platform
API ignore the value and continue to return the default value.

Add a platform ops to retrieve platform specific tlb range flush limit.
The platform variable becomes redundant in presence of the platform ops.
Take this opportunity to remove the variable as well.

The default is still set to smallest page size in RISC-V (4KB), as there
is no way to figure out a best value for all platforms. Individual platform
should set it to the optimal value for their platform.

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