Add support for 16-bit registers

The current i2c-omap driver is set up for 32-bit registers, which
corresponds to most OMAP devices.  However, OMAP730/850 based
devices use a 16-bit register size.

This change modifies the driver to perform a runtime CPU type check
to determine the register sizes, and uses a bit shift of either 1
or 2 bits to compute the proper register sizes for all registers.

Signed-off-by: Cory Maccarrone <darkstar6262@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>