mhi: controller: qcom: add a boot monitor

Repurpose existing async boot log thread as a general purpose boot
monitor for boot status.

CRs-Fixed: 2418347
Change-Id: I7c1391ae9506c13c5f1a17457a2c9c0582089546
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>