drivers: qcom: Add support for kernel boot markers

The existing boot_stats module captures the timestamps
of various events before the kernel start, like bootloader
start, kernel load time, etc.

Extend support to capture timestamps of events after kernel
starts execution. Users can call place_marker from different
points of execution to capture the timestamp.

Expose a sysfs node to read the complete list of events
captured by the boot_stats module.

Change-Id: Icbc2341b34fe7e2559a37c092a5ab74919703184
Signed-off-by: Shreyas K K <shrekk@codeaurora.org>