fsl-ppc/u-boot: Add --bss-plt to LDFLAGS

Recent toolchains compile with a new BSS PLT and Global Offset Table
layout.  However, u-boot.lds has not been updated to handle this.
Adding --bss-plt to LDFLAGS reverts to the old layout.

This was reported in SB-2736.

Signed-off-by: Wade Farnsworth <wade_farnsworth@mentor.com>