hwrng: cavium - Check health status while reading random data

This RNG device is present on Marvell OcteonTx2 silicons as well and
also provides entropy health status.

HW continuously checks health condition of entropy and reports
faults. Fault is in terms of co-processor cycles since last fault
detected. This doesn't get cleared and only updated when new fault
is detected. Also there are chances of detecting false positives.
So to detect a entropy failure SW has to check if failures are
persistent ie cycles elapsed is frequently updated by HW.

This patch adds support to detect health failures using below algo.
1. Consider any fault detected before 10ms as a false positive and ignore.
   10ms is chosen randomly, no significance.
2. Upon first failure detection make a note of cycles elapsed and when this
   error happened in realtime (cntvct).
3. Upon subsequent failure, check if this is new or a old one by comparing
   current cycles with the ones since last failure. cycles or time since
   last failure is calculated using cycles and time info captured at (2).

HEALTH_CHECK status register is not available to VF, hence had to map
PF registers. Also since cycles are in terms of co-processor cycles,
had to retrieve co-processor clock rate from RST device.

Signed-off-by: Sunil Goutham <sgoutham@marvell.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>