hwrng: nomadik - Constify nmk_rng_ids[]

nmk_rng_ids[] is not modified and can be made const to allow the
compiler to put it in read-only memory.

Before:
   text    data     bss     dec     hex filename
    652     216       4     872     368 drivers/char/hw_random/nomadik-rng.o

After:
   text    data     bss     dec     hex filename
    676     192       4     872     368 drivers/char/hw_random/nomadik-rng.o

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>