hwrng: msm: switch Kconfig to ARCH_QCOM depends

We've split Qualcomm MSM support into legacy and multiplatform.  The RNG
driver is only relevant on the multiplatform supported SoCs so switch the
Kconfig depends to ARCH_QCOM.

Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
CC: Stanimir Varbanov <svarbanov@mm-sol.com>
Signed-off-by: Kumar Gala <galak@codeaurora.org>