lib: Move all coldboot wait APIs to sbi_init.c

The coldboot wait APIs are only used by sbi_init.c so no point in
having coldboot related code in sbi_hart.c.

As per-above rationale, we move all coldboot wait related APIs to
sbi_init.c as static/local functions.

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