powerpc/85xx: Add workaround for erratum A-003474

Erratum A-003474: Internal DDR calibration circuit is not supported

Impact:
Experience shows no significant benefit to device operation with
auto-calibration enabled versus it disabled. To ensure consistent timing
results, Freescale recommends this feature be disabled in future customer
products. There should be no impact to parts that are already operating
in the field.

Workaround:
Prior to setting DDR_SDRAM_CFG[MEM_EN]=1, do the following:
1. Write a value of 0x0000_0015 to the register at offset
	CCSRBAR + DDR OFFSET + 0xf30
2. Write a value of 0x2400_0000 to the register at offset
	CCSRBAR + DDR OFFSET + 0xf54

Signed-off-by: York Sun <yorksun@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>