drivers/wdt: Rename CMSDK driver to follow naming rules

Basically, all driver file names should start with the driver type as
prefix: wdt_ in case of watchdogs here, and not something custom like
wdog_.

For clarity, wdog_ prefix could be changed to wdt_ in the source code
also but that's a detail and will not be addressed here.

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>