overlays: Fix compatible string for ds1307 RTC

Kernels since 4.19 have required the correct manufacture name in the
compatible string for I2C devices, and unfortunately the one for the
Dallas/Maxim DS1307 should have been "dallas,ds1307" and not
"maxim,ds1307".

See: https://github.com/raspberrypi/linux/issues/3013



Signed-off-by: Phil Elwell <phil@raspberrypi.org>