ARM: OMAP: Make plat-omap/i2c.c port checks local

The common code should not have any omap1 or omap2+
specific code, and should not need to call the cpu_is_omap
macros.

The only remaining user for cpu_is_omap macros is
omap_i2c_nr_ports(). Let's make those checks in
the omap specific implementation of omap_i2c_add_bus()
instead in order to remove cpu_is_omap usage from
the common code.

Signed-off-by: Tony Lindgren <tony@atomide.com>