hwrng: bcm2835 - fix oops when rng h/w is accessed during registration

Commit "d9e79726 hwrng: add randomness to system from rng sources"
exposed a bug in the bcm2835-rng driver resulting in boot failure
on Raspberry Pi due to the following oops:

[   28.261523] BUG: soft lockup - CPU#0 stuck for 23s! [swapper:1]
[   28.271058]
[   28.275958] CPU: 0 PID: 1 Comm: swapper Not tainted 3.14.0+ #11
[   28.285374] task: db480000 ti: db484000 task.ti: db484000
[   28.294279] PC is at bcm2835_rng_read+0x28/0x48
[   28.302276] LR is at hwrng_register+0x1a8/0x238
.
.
.

The RNG h/w is not completely initialized and enabled before
hwrng_register() is called and so the bcm2835_rng_read() fails.
Fix this by making the warmup/enable writes before registering
the RNG source with the hwrng core.

Signed-off-by: Matt Porter <mporter@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>