hwrng: exynos - Enable COMPILE_TEST

Get some build coverage of Exynos H/W random number generator
driver. Driver uses devm_ioremap_resource() so add IOMEM dependency for
the compile testing.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>