qemu: arm: Remove platform specific function to get RNG device

The QEMU platform has a function defined to get the random number
generator(RNG) device. However, the RNG device can be obtained simply
by searching for a device belonging to the RNG uclass. Remove the
superfluous platform function defined for the QEMU platform for
getting the RNG device.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Tested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>