Kconfig: Increase the pre-relocation memory

The memory is close to full and adding a syscon node in test.dts makes
it go over the limit and makes malloc() fail on startup.

Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

Kconfig

@@ -196,7 +196,7 @@ config SYS_MALLOC_F_LEN
 	hex "Size of malloc() pool before relocation"
 	depends on SYS_MALLOC_F
 	default 0x1000 if AM33XX
-	default 0x2800 if SANDBOX
+	default 0x4000 if SANDBOX
 	default 0x2000 if (ARCH_IMX8 || ARCH_IMX8M || ARCH_MX7 || \
 			   ARCH_MX7ULP || ARCH_MX6 || ARCH_MX5 || \
 			   ARCH_LS1012A || ARCH_LS1021A || ARCH_LS1043A || \