rtc-rs5c372 becomes a new-style i2c driver

Convert rtc-rs5c372 to be a "new style" I2C driver, and update the
Kconfig text to be more complete..

Verified on an OMAP H4 development platform, along with a board
init patch to declare its rv5c387a device.

Only one defconfig -- powerpc/linkstation -- uses this driver; but
several other platforms use it, just without defconfig support.

Such platforms need to be converted so (a) their I2C adapter driver
supports new-style drivers, and (b) board init code declares this
I2C device.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Cc: Voipio Riku <Riku.Voipio@movial.fi>
Acked-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Cc: Martin Michlmayr <tbm@cyrius.com>
Cc: Jean Delvare <khali@linux-fr.org>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>