hwrng: omap - Fix included header from 'asm'

This commit fixes the checkpatch warning:
WARNING: Use #include <linux/io.h> instead of <asm/io.h>
drivers/char/hw_random/omap-rng.c:34

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>