x86, mrst: Add explanation for using 1960 as the year offset for vrtc

Explain the reason for the apparently odd choice of year offset so we don't
get more questions about it.

Signed-off-by: Feng Tang <feng.tang@intel.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
LKML-Reference: <20101117121050.9998.89348.stgit@localhost.localdomain>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>