x86: devicetree: Add missing early_init_dt_setup_initrd_arch stub

This patch fixes the following build failure:

drivers/built-in.o: In function `early_init_dt_check_for_initrd':
/home/florian/dev/kernel/x86/linux-2.6-x86/drivers/of/fdt.c:571:
undefined reference to `early_init_dt_setup_initrd_arch'
make: *** [.tmp_vmlinux1] Error 1

which happens as soon as we enable initrd support on a x86 devicetree
platform such as Intel CE4100.

Signed-off-by: Florian Fainelli <ffainelli@freebox.fr>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Cc: Maxime Bizon <mbizon@freebox.fr>
Acked-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Cc: stable@kernel.org # 2.6.39
Link: http://lkml.kernel.org/r/201106061015.50039.ffainelli@freebox.fr
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>