panther: Set default interrupt value for Environmental Controller

This writes the default value to the register, but it gets rid of
the error that disturbs some of our tests:

ERROR: PNP: 002e.4 70 irq size: 0x0000000001 not assigned
(panther port of Ieab1c776b553c996a7d06e4059110943aaf41338)

BRANCH=none
BUG=chrome-os-partner:23945
TEST=boot test on Panther

Change-Id: Id45c3bdc0d2feaf6f75d984c41d1f6ffef592d4d
Reviewed-on: https://chromium-review.googlesource.com/177468
Reviewed-by: Stefan Reinauer <reinauer@chromium.org>
Commit-Queue: Mohammed Habibulla <moch@google.com>
Tested-by: Mohammed Habibulla <moch@google.com>

src/mainboard/google/panther/devicetree.cb

@@ -110,6 +110,7 @@ chip northbridge/intel/haswell
 					device pnp 2e.4 on # Environment Controller
 						io 0x60 = 0x700
 						io 0x62 = 0x710
+						irq 0x70 = 0x09
 						irq 0xfa = 0x12
 					end
 					device pnp 2e.7 on # GPIO