net: ipa: introduce ipa_interrupt_init()

Create a new function ipa_interrupt_init() that is called at probe
time to allocate and initialize the IPA interrupt data structure.
Create ipa_interrupt_exit() as its inverse.

This follows the normal IPA driver pattern of *_init() functions
doing things that can be done before access to hardware is required.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>