rtc: m41t62: add compatible for m41st87

This adds a compatible string for m41st87. This ensures that this driver
can be used for m41st87.

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