ltq-adsl: fix compilation with Linux 5.0+

ltq-adsl: fix compilation with Linux 5.0+

Upstream commit e4b92b108c6cd6 ("timekeeping: remove obsolete time
accessors") removed do_gettimeofday(). In Linux 4.19 this was only a
wrapper around ktime_get_real_ts64(). Use ktime_get_real_ts64() now that
the wrapper is gone to fix compilation against Linux 5.4.

Move the ifxmips_mei_interface header to the include directory, it can't
be found otherwise during compilation. The reason for the changed
behaviour is not yet clear, however having header files in an include
directory is more straight forward.

To use the of_device_id struct, the mod_devicetable header need to be
included. Instead of including this header, include the of_platform
header, which includes the mod_devicetable on its own.

Signed-off-by: Mathias Kresin <dev@kresin.me>