lib: Introduce bitmap to track HARTs waiting for coldboot

On QEMU Virt, max supported HARTs are 8 but number of HARTs
actually depend on "-smp" command-line parameter passed to
QEMU. This creates problems in sbi_hart_wake_coldboot_harts()
because when number of HARTs are less than 8.

To tackle this, we introduce a bitmap to track HARTs waiting
for coldboot to finish. We wake only those HARTs who have
set their bit in coldboot bitmap.

Signed-off-by: Anup Patel <anup.patel@wdc.com>