rtc: abx80x: handle autocalibration

The autocalibration is separated in two bits to set in Oscillator
Control register (0x1c) :
 - OSEL bit to select the oscillator type (XT or RC).
 - ACAL bit to select the autocalibration type.

These functionnalities are exported in sysfs entries : "oscillator"
and "autocalibration". Respectively, the values are "xtal" for XT
oscillator and "rc" for RC oscillator and 0 to disable the
autocalibration cycle, 512 for a 512 seconds autocalibration cycle
and 1024 for a cycle of 1024 seconds.

Examples :
Set to XT Oscillator
echo xtal > /sys/class/rtc/rtc0/device/oscillator
Activate an autocalibration every 512 seconds
echo 512 > /sys/class/rtc/rtc0/device/autocalibration

Signed-off-by: Mylène Josserand <mylene.josserand@free-electrons.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>