rtc: m41t80: add clock provider support

Some devices supported by the m41t80 driver have a programmable
square-wave output signal (see M41T80_FEATURE_SQ).

This enables to use this feature as a clock provider of common
clock framework.

Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>