ARM: am3517_evm: Build for Thumb

ARM: am3517_evm: Build for Thumb

In an effort to free up more resources in SPL and U-Boot, building
for Thumb shrinks the code side.

Before:

  text	   data	    bss	    dec	    hex	filename
  685588  25808	 275724	 987120	  f0ff0	u-boot

  text	   data	    bss	    dec	    hex	filename
  55324	    417	  67460	 123201	  1e141	spl/u-boot-spl

After:

   text	   data	    bss	    dec	    hex	filename
 515502	  25808	 275708	 817018	  c777a	u-boot

   text	   data	    bss	    dec	    hex	filename
  42910	    417	  67460	 110787	  1b0c3	spl/u-boot-spl

Signed-off-by: Adam Ford <aford173@gmail.com>