drivers/rtc/rtc-omap.c: include <linux/io.h> instead of <asm/io.h>

Use #include <linux/io.h> instead of <asm/io.h> as pointed out by
checkpatch.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Cc: George G. Davis <gdavis@mvista.com>
Cc: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>