lib: sbi_hsm: Don't use sbi_platform_hart_count() API

We remove usage of sbi_platform_hart_count() API from sbi_hsm
so that discontinuous and sparse HART ids can be supported.

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