x86: devicetree: fix config option around x86_flattree_get_config()

x86: devicetree: fix config option around x86_flattree_get_config()

x86_flattree_get_config() is incorrectly protected by
ifdef CONFIG_OF_FLATTREE.  It uses of_get_flat_dt_size(), which
only exists if CONFIG_OF_EARLY_FLATTREE.  This issue has not
been exposed previously because OF_FLATTREE did not occur unless
it was selected by OF_EARLY_FLATTREE.  A devicetree overlay change
is selecting OF_FLATTREE directly instead of indirectly enabling
it by selecting OF_EARLY_FLATTREE.

This problem was exposed by a randconfig generated by the kbuild
test robot, where Platform OLPC was enabled.  OLPC selects
OF_PROMTREE instead of OF_EARLY_FLATREE.  The only other x86
platform that selects OF is X86_INTEL_CE, which does select
OF_EARLY_FLATTREE.

Signed-off-by: Frank Rowand <frank.rowand@sony.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>