hwrng: timeriomem - relax check on memory resource size

The timeriomem_rng driver only accesses the first 4 bytes of the given
memory area and currently, it also forces that memory resource to be
exactly 4 bytes in size.

This, however, is problematic when used with device-trees that are
generated from things like FPGA toolchains, where the minimum size
of an exposed memory block may be something like 4k.

Hence, let's only check for what's needed for the driver to operate
properly; namely that we have enough memory available to read the
random data from.

Signed-off-by: Daniel Mack <daniel@zonque.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>