mb/google/poppy/variants/nami: Fix Elan touchscreen power off sequence

Power off does not seem to use the ACPI _OFF function, but rather the
smihandler.  Creating variant_smi_sleep function for nami to handle
the power off sequence during reboot/power off.

BUG=b:78311818
BRANCH=None
TEST=Run "poweroff" command from AP console with SMI_DEBUG enabled
     Make sure delays are consistent with spec

Change-Id: Ifeea545fe268be249793b3e508c51f5e4c1a3460
Signed-off-by: Shelley Chen <shchen@google.com>
Reviewed-on: https://review.coreboot.org/26724
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>
4 files changed