hwrng: atmel - add new platform support for sam9x60

Add platform support for the new IP found on sam9x60 SoC. For this
version, if the peripheral clk is above 100MHz, the HALFR bit must be
set. This bit is available only if the IP can generate a random number
every 168 cycles (instead of 84).

Signed-off-by: Codrin Ciubotariu <codrin.ciubotariu@microchip.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>