meta-ampere: mtmitchell: update u-boot device tree

Update UBOOT_DEVICETREE from ast2600-bletchley to ast2600-ampere dtb

Signed-off-by: Chanh Nguyen <chanh@os.amperecomputing.com>
Change-Id: Idb9935f693974326e4ca7cd64dce0c713015b4e4
diff --git a/meta-ampere/meta-mitchell/conf/machine/mtmitchell.conf b/meta-ampere/meta-mitchell/conf/machine/mtmitchell.conf
index 24b24dd..93e4a26 100644
--- a/meta-ampere/meta-mitchell/conf/machine/mtmitchell.conf
+++ b/meta-ampere/meta-mitchell/conf/machine/mtmitchell.conf
@@ -1,6 +1,6 @@
 KMACHINE = "aspeed"
 KERNEL_DEVICETREE = "aspeed/${KMACHINE}-bmc-ampere-mtmitchell.dtb"
-UBOOT_DEVICETREE = "ast2600-bletchley"
+UBOOT_DEVICETREE = "ast2600-ampere"
 UBOOT_MACHINE = "ast2600_openbmc_spl_defconfig"
 SPL_BINARY = "spl/u-boot-spl.bin"
 SOCSEC_SIGN_ENABLE = "0"