rtc: m48t86: verify that the RTC is actually present

The RTC is an optional feature at purchase time on some Technologic
Systems boards. Verify that it actually exists by checking if the
last two bytes of the NVRAM can be changed.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>