ppc4xx: Fixed offset of refresh rate type for Bamboo on-board DDR SDRAM

This patch also adds a note to the fixed DDR setup for Bamboo NAND booting:

Note:
As found out by Eugene O'Brien <eugene.obrien@advantechamt.com>, the fixed
DDR setup has problems (U-Boot crashes randomly upon TFTP), when the DIMM
modules are still plugged in. So it is recommended to remove the DIMM
modules while using the NAND booting code with the fixed SDRAM setup!

Signed-off-by: Eugene O'Brien <eugene.obrien@advantechamt.com>
Signed-off-by: Stefan Roese <sr@denx.de>