lib: sbi: Simplify wait_for_coldboot() implementation

On QEMU virt machine with large number of HARTs, some of the HARTs
randomly fail to come out of wait_for_coldboot() due to one of the
following race-conditions:

1) Failing HARTs are not able to acquire the coldboot_lock and
   update the coldboot_hartmask in wait_for_coldboot() before
   the coldboot HART acquires the coldboot_lock and sends IPI
   in wake_coldboot_harts() hence the failing HARTs never
   receive IPI from the coldboot HART.

2) Failing HARTs acquire the coldbood_lock and update the
   coldboot_hartmask before coldboot HART does sbi_scratch_init()
   so the sbi_hartmask_set_hartid() does not update the
   coldboot_hartmask on the failing HARTs hence they never
   receive IPI from the coldboot HART.

To address this, use a simple busy-loop in wait_for_coldboot() for
polling on coldboot_done flag.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>