Create a new function gsi_irq_ev_ctrl_enable() that encapsulates
enabling the event ring control GSI interrupt type, and enables a
single event ring to signal that interrupt.  When an event ring
changes state as a result of an event ring command, it triggers this
interrupt.

Create an inverse function gsi_irq_ev_ctrl_disable() as well.
Because only one event ring at a time is enabled for this interrupt,
we can simply disable the interrupt for *all* channels.

Create a pair of helpers that serve the same purpose for channel
commands.
Signed-off-by: Alex Elder <elder@linaro.org>
Reviewed-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>