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>

Documentation/ABI/testing/sysfs-bus-i2c-devices-bq32k

@@ -0,0 +1,7 @@
+What:		/sys/bus/i2c/devices/.../trickle_charge_bypass
+Date:		Jan 2017
+KernelVersion:	4.11
+Contact:	Enric Balletbo i Serra <eballetbo@gmail.com>
+Description:    Attribute for enable/disable the trickle charge bypass
+		The trickle_charge_bypass attribute allows the userspace to
+                enable/disable the Trickle charge FET bypass.