hwrng: imx-rngc - improve dependencies

The imx-rngc driver binds to devices that are compatible to
"fsl,imx25-rngb". Grepping through the device tree sources suggests this
only exists on i.MX25. So restrict dependencies to configs that have
this SoC enabled, but allow compile testing. For the latter additional
dependencies for clk and readl/writel are necessary.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>