ARM: omapl138_lcdk: Shrink code size by building with Thumb

SPL has limited available resources, and the performance between
ARM and Thumb isn't that significant.

This patch builds using Thumb instruction set to reduce the code
size by nearly 6K.

Original:
   text	   data	    bss	    dec	    hex	filename
  26526	   4004	   1376	  31906	   7ca2	spl/u-boot-spl

Thumb:

   text	   data	    bss	    dec	    hex	filename
  20232	   4004	   1376	  25612	   640c	spl/u-boot-spl

Signed-off-by: Adam Ford <aford173@gmail.com>
Tested-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>