arm: config: omap2plus_defconfig: switch over to LZMA compression

LZMA compression makes about 33% smaller zImage
with just a slight extra decompression time.

Before this patch, zImage built with o2+_dc
is 4.5MiB and after it's about 3.3MiB.

Suggested-by: David Cohen <david.a.cohen@linux.intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>