x86: dt: Fix OLPC=y/INTEL_CE=n build

Both OLPC and CE4100 activate CONFIG_OF. OLPC uses PROMTREE while CE
uses FLATTREE. Compiling for OLPC only breaks due to missing flat tree
functions and variables.

Use proper wrappers and provide an empty x86_flattree_get_config()
inline so OF=y FLATTREE=n builds and works.

[ tglx: Make it work with HPET_TIMER=n and make a function static ]

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>