platform/x86: mlx-platform: Move module from arch/x86

Since mlx-platform is not an architectural driver, it is moved out
of arch/x86/platform to drivers/platform/x86.
Relevant Makefile and Kconfig are updated.

Signed-off-by: Vadim Pasternak <vadimp@mellanox.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>