hwrng: bcm2835 - Switch to SPDX identifier

Adopt the SPDX license identifier headers to ease license compliance
management. While we are at this fix the comment style, too.

Cc: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Acked-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>