soc: qcom: ramdump: Synchronize dump reading for multiple readers

When multiple threads are reading the ramdump collected for a
particular device, the first thread to finish reading the
entire ramdump will wake up the thread that is waiting for
the ramdump to be read, allowing it to move forward. This
can cause an issue, as the other threads that are still
reading the ramdump have references to memory that may
change--but shouldn't--or device memory that may be unmapped
from the HLOS.

Synchronize the ramdump API routines to ensure that the
thread that is waiting for the ramdump to be read does not
proceed, unless all other consumer threads are done reading
the ramdump.

Change-Id: Ib442e4b361494eafbb26e0191e0f5749c3396043
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>