arm: mvebu: Update docs for Allied Telesis x220

The current documentation doesn't work as-is for extracting the DDR
training binary from the vendor bootloader. There are some seemingly
undocumented alignment requirements that we need to meet by padding the
binary file. Add some instructions on how to do this with objcopy. Also
fix a minor spelling mistake while we're here.

Signed-off-by: Chris Packham <[email protected]>
Reviewed-by: Stefan Roese <[email protected]>
[sr: fix title underline length]

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