keystone2: ecc: add ddr3 error detection and correction support

This patch adds the DDR3 ECC support to enable ECC in the DDR3
EMIF controller for Keystone II devices.

By default, ECC will only be enabled if RMW is supported in the
DDR EMIF controller. The entire DDR memory will be scrubbed to
zero using an EDMA channel after ECC is enabled and before
u-boot is re-located to DDR memory.

An ecc_test environment variable is added for ECC testing.
If ecc_test is set to 0, a detection of 2-bit error will reset
the device, if ecc_test is set to 1, 2-bit error detection
will not reset the device, user can still boot the kernel to
check the ECC error handling in kernel.

Signed-off-by: Hao Zhang <hzhang@ti.com>
Signed-off-by: Vitaly Andrianov <vitalya@ti.com>
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@ti.com>