mb/google/puff: Enable cros_ec_keyb device

This is required to transmit button information from EC to kernel.

BUG=b:150830342
BRANCH=None
TEST=firmware_ECPowerButton test passes on puff

Change-Id: I10ba9d55e8997ce2412deb0613cfcaa8f24f271d
Signed-off-by: Kangheui Won <khwon@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/39391
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: Edward O'Callaghan <quasisec@chromium.org>

src/mainboard/google/hatch/variants/puff/include/variant/ec.h

@@ -51,6 +51,9 @@
  * ACPI related definitions for ASL code.
  */
 
+/* Enable cros_ec_keyb device */
+#define EC_ENABLE_MKBP_DEVICE
+
 /* Enable EC backed PD MCU device in ACPI */
 #define EC_ENABLE_PD_MCU_DEVICE