x86: irq: Support flags for acpi_gpe

This binding currently has a flags cell but it is not used. Make use of it
to create ACPI tables for interrupts.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Wolfgang Wallner <wolfgang.wallner@br-automation.com>