serial: lantiq: Replace lantiq_soc.h with lantiq.h

In this existing lantiq serial driver,
lantiq_soc.h is defined in the arch directory,

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

This driver need to be extended to support more platform,
lantiq.h is added in include/linux/ to make it
globally available and provide some wrapper code.
Use lantiq.h to make the driver can find the correct
header file.

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