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>

drivers/char/hw_random/Kconfig

@@ -244,7 +244,8 @@ config HW_RANDOM_MXC_RNGA
 
 config HW_RANDOM_IMX_RNGC
 	tristate "Freescale i.MX RNGC Random Number Generator"
-	depends on ARCH_MXC
+	depends on HAS_IOMEM && HAVE_CLK
+	depends on SOC_IMX25 || COMPILE_TEST
 	default HW_RANDOM
 	---help---
 	  This driver provides kernel-side support for the Random Number