msm: pil: bootkpi: Add conditional boot marker for modem

Add conditional boot marker for modem sub-system in peripheral
loader driver. Condition is added to find subsystem
and place marker only for modem subsystem.

Signed-off-by: Remil Ram <remilr@codeaurora.org>
Change-Id: I5697b78e0a8322477e78d7b5f9814e1c18b42152