Introduce gsi_irq_setup() and gsi_irq_teardown() to disable all
GSI interrupts when first setting up GSI hardware, and to clean
things up when we're done.

Re-enable all GSI interrupt types in gsi_irq_enable(), but do
so only after each of the type-specific interrupt masks has
been configured.  Similarly, disable all interrupt types in
gsi_irq_disable()--first--before zeroing out the type-specific
masks.
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>