rtc: m48t86: shorten register name defines

For aesthetics. Shorten all the register names by removing '_REG' from all
of them.
This helps fix all the checkpatch.pl issues.

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