rtc: m41t62: add compatible for m41t82

This adds a compatible string for m41t82. This ensures that this driver
can be used for m41t82 in DM mode, too (asit was usable for this model in
non-DM mode before).

In addition, the HT bit has to be reset during probe, since the m41t82
chip sets it when entering battery standby mode.

This patch ensures this driver works on socfpga_socrates.

Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>