Implement the generic acpi_get_gpe() function to read and clear the GPE status for a specific GPE. Tested by watching GPE status in a loop while generating interrupts manually from the EC console. BUG=chrome-os-partner:53336 Change-Id: Id885e98d48c2133a868da19eca3360e2dfb82e84 Signed-off-by: Duncan Laurie <dlaurie@chromium.org>