include: Add lantiq.h in include/linux/

In some existing lantiq driver, the C codes include lantiq_soc.h
header file directly.

./arch/mips/include/asm/mach-lantiq/falcon/lantiq_soc.h
./arch/mips/include/asm/mach-lantiq/xway/lantiq_soc.h

Those drivers need to be extended to support more platform.
lantiq.h is added in include/linux/ to make it
globally available and provides some wrapper codes.

Signed-off-by: Songjun Wu <songjun.wu@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>