i2c: dm: Record maximum id of devices before probing devices

There is a need to find out the first free i2c ID which can be used for
i2s buses (including i2c buses connected to i2c mux). Do it early in
init and share this variable with other i2c classes for uniq bus
identification.

add from hs:
fix build problem in i2c-uclass.c for omap devices

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Heiko Schocher <hs@denx.de>