[ARM] pxa: prepare xm_x2xx_defconfig for split

xm_x2xx_defconfig currently supports 3 platforms: CM-X255, CM-X270 and
EM-X270. Although EM-X270 is similar to CM-X2XX, it has a lot of unique
features. Keeping these features in the same _defconfig increases the
kernel size in the way it does not fit into CM-X2XX NOR flash.
Rename xm_x2xx_defconfig to cm_x2xx_defconfig and remove EM-X270 specifc
parts from it.
Signed-off-by: Mike Rapoport <mike@compulab.co.il>
Signed-off-by: Eric Miao <eric.miao@marvell.com>