x86: add kernel module for sp5100_tco watchdog

x86: add kernel module for sp5100_tco watchdog

This change adds the sp5100_tco driver as a kernel module for the x86
target. Specifically, this can be used by the PCEngines APU2/APU3. The
reason for having this as a kernel module is to allow users to
load/unload it on demand, as the I2C interface on the APU2/APU3 will not
work while this module is loaded. More info can be found on GitHub at
https://github.com/riptidewave93/LEDE-APU2/pull/5#issuecomment-255667736

Signed-off-by: Chris Blake <chrisrblake93@gmail.com>