Create new function ipa_interrupt_enable() to encapsulate enabling
one of the IPA interrupt types.  Introduce ipa_interrupt_disable()
to reverse that operation.  Add a helper function to factor out the
common register update used by both.

Use these in ipa_interrupt_add() and ipa_interrupt_remove().
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>