msm: ipa: add support to configure WAN_RX pool size

This parameter is target specific and should be configured
through device tree. This commit adds that support.

Change-Id: Ib6e6a64b7fd3ce0c544b7b4cb225ccb593877666
Signed-off-by: Ravi Gummadidala <rgummadi@codeaurora.org>