hwrng: omap - Add support for 128-bit output of data

So far, this driver only supports up to 64 bits of output data generated
by an RNG. Some IP blocks, like the SafeXcel IP-76 supports up to 128
bits of output data. This commits renames registers descriptions
OUTPUT_L_REG and OUTPUT_H_REG to OUTPUT_0_REG and OUPUT_1_REG,
respectively. It also adds two new values to the enumeration of existing
registers: OUTPUT_2_REG and OUTPUT_3_REG.

Signed-off-by: Romain Perier <romain.perier@free-electrons.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>