[ARM] 5036/2: Combine cm_x270_defconfig and em_x270_defconfig into xm_x270_defconfig

Combine cm_x270_defconfig and em_x270_defconfig to allow running the
same kernel on both machines
Signed-off-by: NMike Rapoport <mike@compulab.co.il>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>