driver/ddr/fsl: Add built-in memory test for DDR4 driver

Add built-in memory test to catch errors after DDR is initialized, before
any other transactions. To enable this test, define CONFIG_FSL_DDR_BIST.
An environmental variable "ddr_bist" is checked before starting test.
It takes a while (several seconds) depending on system memory size.

Signed-off-by: York Sun <yorksun@freescale.com>
diff --git a/README b/README
index d33f3b5..f5a719d 100644
--- a/README
+++ b/README
@@ -4884,6 +4884,9 @@
 - CONFIG_FSL_DDR_SYNC_REFRESH
 		Enable sync of refresh for multiple controllers.
 
+- CONFIG_FSL_DDR_BIST
+		Enable built-in memory test for Freescale DDR controllers.
+
 - CONFIG_SYS_83XX_DDR_USES_CS0
 		Only for 83xx systems. If specified, then DDR should
 		be configured using CS0 and CS1 instead of CS2 and CS3.