rtc: bq32000: add support to enable disable the trickle charge FET bypass

rtc: bq32000: add support to enable disable the trickle charge FET bypass

The bq32000 includes a trickle charge circuit to maintain the charge of the
backup supply when a super capacitor is used.

You can enable the charging circuit by setting 'trickle-resistor-ohms',
additionally you can set TCFE to 1 to bypass the internal diode and boost
the charge voltage of the backup supply. You might want to enable/disable
the TCFE switch from userspace (e.g when device is only connected to a
battery)

This patch introduces a new sysfs entry to enable and disable this FET
form userspace.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>