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>