ARM: OMAP: Remove unnecessary mach and plat includes

Now mach/hardware.h is empty for omap2+ and can be
removed except for plat-omap/dmtimer.c for omap1.

Also the include of mach/irqs.h can now be removed
for shared plat-omap/i2c.c as it's no longer needed.

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