msm: ipa3: create ipa_clients folder and add USB client

As part of of IPA driver refactoring a separation has been made
between IPA core driver and the IPA clients.
A new folder named ipa_clients has been created and ipa_usb.c
has become a separate driver.
IPA clients drivers are the interface between IPA core driver
and external drivers. Specifically, ipa_usb driver is the
interface between the USB driver and IPA core.

CRs-Fixed: 971511
Change-Id: Ic8e78733cf8c0da441278e261236a6e61027eec9
Signed-off-by: Amir Levy <alevy@codeaurora.org>