crypto: exynos - Icrease the priority of the driver

exynos-rng is one of many implementations of stdrng. With priority as
low as 100 it isn't selected, if software implementations (DRBG) are
available. The value 300 was selected to give the PRNG priority before
software implementations, but allow them to be selected in FIPS-mode
(fips=1 in the kernel command line).

Signed-off-by: Łukasz Stelmach <l.stelmach@samsung.com>
Reviewed-by: Stephan Mueller <smueller@chronox.de>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>