drivers: qcom: Add boot stat entries for kernel load start and end

The bootloader adds IMEM entries for kernel load start and end
timestamps. Capture the same in the kernel as part of boot stats.
This allows to measure the Bootloader KPI values.

Change-Id: I2c1862d0f662a6d59aed7bb40e1d7d84ef686c14
Signed-off-by: Shreyas K K <shrekk@codeaurora.org>