soc: qcom: mem-buf: Use an unbound high priority workqueue for mem-buf

Use an unbound high priority workqueue to ensure that mem-buf
transactions can be handled on any CPU, as well as with high
priority. Also, increase the timeout for allocation responses
from 2 seconds to 3.5 seconds.

Change-Id: I2ec4d85bf7307d64d2f8a7dae6dfb2b29cfc45bb
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>