driver core: platform: Add platform_get_irq_byname_optional()

Some drivers (e.g dwc3) first try to get an IRQ byname and then fall
back to the one at index 0. In this case we do not want the error(s)
printed by platform_get_irq_byname(). This commit adds a new
platform_get_irq_byname_optional(), which does not print errors, for this.

While at it also improve the kdoc text for platform_get_irq_byname() a bit.

BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=205037
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://lore.kernel.org/r/20191005210449.3926-2-hdegoede@redhat.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>