Exclude drivers/qe/fdt.c without CONFIG_QE

Previously with archive libraries fdt.o was compiled and included in
qe.a and then discarded by the linker.  With partial linking this
results in unresolved symbols, which this commit fixes.

This commit also cleans up a now-useless conditional in fdt.c.

Signed-off-by: Sebastien Carlier <sebastien.carlier@gmail.com>
Tested-by: Wolfgang Denk <wd@denx.de>