mfd: Add mc13892 support to mc13xxx

mc13892 is the companion PMIC for Freescale's i.MX51.  It's similar enough
to mc13782 to support it in a single driver.

This patch introduces enough compatibility cruft to keep all users of the
superseded mc13783 driver unchanged.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>