hwrng: bcm2835 - Constify bcm2835_rng_devtype[]

bcm2835_rng_devtype[] 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
   2392     176       0    2568     a08 drivers/char/hw_random/bcm2835-rng.o

After:
   text    data     bss     dec     hex filename
   2464     104       0    2568     a08 drivers/char/hw_random/bcm2835-rng.o

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>