x86/platform/intel-mid: Rename platform_wdt to platform_mrfld_wdt

x86/platform/intel-mid: Rename platform_wdt to platform_mrfld_wdt

Rename the watchdog platform library file to explicitly show that is used only
on Intel Merrifield platforms.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20161118172723.179761-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>