hwrng: bcm2835 - Rework interrupt masking

The interrupt masking done for Northstart Plus and Northstar (BCM5301X)
is moved from being a function pointer mapped to of_device_id::data into
a proper part of the hwrng::init callback. While at it, we also make the
of_data be a proper structure indicating the platform specifics, since
the day we need to add a second type of platform information, we would
have to do that anyway.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>