skylake: Add support for eSPI SMI events

Add the necessary infrastructure to support eSPI SMI events,
and a mainboard handler to pass control to the EC.

BUG=chrome-os-partner:58666
TEST=tested on eve board with eSPI enabled, verified that lid
close event from the EC during firmware will result in an SMI
and shut down the system.

Change-Id: I6367e233e070a8fca053a7bdd2534c0578d15d12
Signed-off-by: Duncan Laurie <dlaurie@chromium.org>
Reviewed-on: https://review.coreboot.org/17134
Tested-by: build bot (Jenkins)
Reviewed-by: Furquan Shaikh <furquan@google.com>

src/soc/intel/skylake/pmutil.c

@@ -177,7 +177,8 @@ static u32 print_smi_status(u32 smi_sts)
 		[20] = "PCI_EXP_SMI",
 		[21] = "MONITOR",
 		[26] = "SPI",
-		[27] = "GPIO_UNLOCK"
+		[27] = "GPIO_UNLOCK",
+		[28] = "ESPI_SMI",
 	};
 
 	if (!smi_sts)