hwrng: cavium - HW_RANDOM_CAVIUM should depend on ARCH_THUNDER

The Cavium ThunderX Random Number Generator is only present on Cavium
ThunderX SoCs, and not available as an independent PCIe endpoint.  Hence
add a dependency on ARCH_THUNDER, to prevent asking the user about this
driver when configuring a kernel without Cavium Thunder SoC  support.

Fixes: cc2f1908 ("hwrng: cavium - Add Cavium HWRNG driver for ThunderX SoC.")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>