include: configs: am64x_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
 144572	   5484	   1752	 151808	  25100	spl/u-boot-spl

But 20KB is allocated currently for BSS. Reduce it to 4KB and
save some space for stack.
Signed-off-by: NLokesh Vutla <lokeshvutla@ti.com>

include/configs/am64x_evm.h

@@ -35,7 +35,7 @@
  * our memory footprint. The less we use for BSS the more we have available
  * for everything else.
  */
-#define CONFIG_SPL_BSS_MAX_SIZE		0x5000
+#define CONFIG_SPL_BSS_MAX_SIZE		0x1000
 /*
  * Link BSS to be within SPL in a dedicated region located near the top of
  * the MCU SRAM, this way making it available also before relocation. Note