omap: use common initialization for PMIC i2c bus

Introduce omap_pmic_init that registers i2c bus and PMIC device on that
bus and add omap2/3/4 wrappers for common cases.

Signed-off-by: Mike Rapoport <mike@compulab.co.il>
Signed-off-by: Tony Lindgren <tony@atomide.com>