x86: Add support for ACPI general-purpose events

ACPI GPEs are used to signal interrupts from peripherals that are accessed
via ACPI. In U-Boot these are modelled as interrupts using a separate
interrupt controller. Configuration is via the device tree.

Add a simple driver for this.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>