swift: Point to swift's kernel device tree

The kernel device tree for swift has been merged upstream.

Tested:
    Output of "bitbake obmc-phosphor-image -e":
    # pre-expansion value:
    #   "${KMACHINE}-bmc-opp-${MACHINE}.dtb"
    KERNEL_DEVICETREE="aspeed-bmc-opp-swift.dtb"

Change-Id: Ie56a243e8635fdb616ed930f0389f77e61271abf
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
1 file changed