x86/init: Allow DT configured systems to disable RTC at boot time

Systems which do not support RTC run into boot problems as the kernel
assumes the availability of the RTC by default.

On device tree configured systems the availability of the RTC can be
detected by querying the corresponding device tree node.

Implement a wallclock init function to query the device tree and disable
RTC if the RTC is marked as not available in the corresponding node.

[ tglx: Rewrote changelog and comments. Added proper __init(const)
  	annotations. ]

Suggested-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Rahul Tanwar <rahul.tanwar@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/b84d9152ce0c1c09896ff4987e691a0715cb02df.1570693058.git.rahul.tanwar@linux.intel.com