uboot-mxs: build with mxs generic subtarget
Fix image build fail by using the generic subtarget.
Fixes: 64ef920b ("mxs: add generic subtarget")
Ref: 40e3f660 ("uboot-fritz4040: build with ipq40xx generic subtarget")
Signed-off-by: Kuan-Yi Li <[email protected]>
[ add commit description ]
Signed-off-by: Christian Marangi <[email protected]>