lib: sbi_init: Use hartmask for coldboot wait

The coldboot_wait_bitmap is of fixed size and has __riscv_xlen
bits. This limits us to scale beyond __riscv_xlen HARTs hence
we replace coldboot_wait_bitmap with coldboot_wait_hmask which
is of type struct sbi_hartmask.

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>