hwrng: mxc-rnga - Fix Kconfig dependency

We can directly depend on SOC_IMX31 since commit c9ee949
("ARM: imx: deconstruct mxc_rnga initialization")

Since that commit, CONFIG_HW_RANDOM_MXC_RNGA could not be switched on
with unknown symbol ARCH_HAS_RNGA and mxc-rnga.o can't be generated with
ARCH=arm make M=drivers/char/hw_random
Previously, HW_RANDOM_MXC_RNGA required ARCH_HAS_RNGA
which was based on IMX_HAVE_PLATFORM_MXC_RNGA  && ARCH_MXC.
IMX_HAVE_PLATFORM_MXC_RNGA  was based on SOC_IMX31.

Fixes: c9ee949 ("ARM: imx: deconstruct mxc_rnga initialization")
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

drivers/char/hw_random/Kconfig

@@ -244,7 +244,7 @@ config HW_RANDOM_TX4939
 
 config HW_RANDOM_MXC_RNGA
 	tristate "Freescale i.MX RNGA Random Number Generator"
-	depends on ARCH_HAS_RNGA
+	depends on SOC_IMX31
 	default HW_RANDOM
 	---help---
 	  This driver provides kernel-side support for the Random Number