include: Remove disabled_hart_mask from sbi_platform

The disabled_hard_mask in sbi_platform is only 64bits wide so we
cannot disable a HART with HARTID > 63. To tackle this, we remove
disabled_hart_mask and replace it with hart_disabled() platform
callback.

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