hwrng: st - use devm_platform_ioremap_resource() to simplify code

Use devm_platform_ioremap_resource() to simplify the code a bit.
This is detected by coccinelle.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>