usb: pd: Fix cyclic dependency of pd modules

The compilation of policy_engine.c and qpnp-pdphy.c as
loadble modules leads to the circular symbol dependency
of one module on the other.
Fix this by serializing this dependency and ensures that
no symbols from the qpnp-pdphy are directly used in
policy_engine driver.
Also, add the dependency of IIO for policy_engine driver.

Change-Id: I965f887e5c0b5a0cd372cc83353380bad5d8c839
Signed-off-by: Sriharsha Allenki <sallenki@codeaurora.org>