skylake: Add GPE header file to chip.h

Add the GPE header file to skylake chip.h so the SOC-defined macros
for the various GPE values can be used in devicetree directly.

For example:
  chip drivers/i2c/touchpad
    register "wake" = "GPE0_DW0_05"
    device i2c 15.0 on end
  end

Change-Id: Ic322108561b34aa34a24a4daba6ba7a4f7a3f9a4
Signed-off-by: Duncan Laurie <dlaurie@chromium.org>
Reviewed-on: https://review.coreboot.org/14991
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/soc/intel/skylake/chip.h

@@ -21,6 +21,7 @@
 
 #include <stdint.h>
 #include <soc/gpio_defs.h>
+#include <soc/gpe.h>
 #include <soc/pci_devs.h>
 #include <soc/pmc.h>
 #include <soc/serialio.h>