powerpc/b4860qds: add workaround for XFI

XFI does not work stable on current board, it's due to heat sink issue,
to make it work stable the board needs additional heat sink, enable two
XFI lanes only. Right now we do not have such an erratum for the issue,
so use a define CONFIG_SYS_FSL_B4860QDS_XFI_ERR to identify it.
The workaround will only be used in XFI protocols and only if the
hwconfig indicates that XFI is prefered.

A new VSC3308 config function is used instead of re-use the original
function, to avoid making the function complex and ugly.

Signed-off-by: Shaohui Xie <Shaohui.Xie@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>