include: configs: am65x_evm: Optimize size of SPL BSS

Current BSS allocation of SPL is as below:
$ size spl/u-boot-spl
   text	   data	    bss	    dec	    hex	filename
 132369	   7852	   1496	 141717	  22995	spl/u-boot-spl

But 20KB is allocated currently for BSS. Reduce it to 3KB and save some
space for stack.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>