driver core: platform: Introduce platform_get_irq_optional()

In some cases the interrupt line of a device is optional. Introduce a
new platform_get_irq_optional() that works much like platform_get_irq()
but does not output an error on failure to find the interrupt.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Link: https://lore.kernel.org/r/20190828083411.2496-1-thierry.reding@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>