platform: Use one unified per-HART stack size macro for all platforms

As of today all platforms use 8KB of per-HART stack hence there is
no need for each platform to define its own macro or use the magic
number. Create one macro for all platforms. Platform still can use
its own version if needed.

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