ABI: rtc-ab8500: fix rtc_calibration documentation

The "What:" field at the ABI should describe the location of
the ABI, e. g. the position under a mounted sysfs.

Fix it.

Cc: Mark Godfrey <mark.godfrey@stericsson.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: rtc-linux@googlegroups.com
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>