hwrng: mxc-rnga - Fix Kconfig dependency

We can directly depend on SOC_IMX31 since commit c9ee9496
("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: c9ee9496

 ("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>