rtc: m48t86: add NVRAM support

This RTC has 114 bytes of NVRAM. Provide access to it via a binary
sysfs 'nvram' attribute file.

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