lib: sbi: Wakeup non-coldboot HARTs early in the coldboot path

Currently, all non-coldboot HARTs busy spin in wait_for_coldboot()
until the entire coldboot init sequence is completed.

This means:
1) On QEMU, all non-coldboot HARTs will eat host CPU time and
   also slow down the coldboot HART until the entire coldboot
   init sequence is completed.
2) On real HW, all non-coldboot HARTs will consume more CPU
   power until the entire coldboot init sequence is completed.

To address this, wake up all non-coldboot HARTs as early as
possible in the coldboot init sequence.

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