hwrng: mxc-rnga - Drop usage of platform_driver_probe()

There are considerations to drop platform_driver_probe() as a concept
that isn't relevant any more today. It comes with an added complexity
that makes many users hold it wrong. (E.g. this driver should have mark
the driver struct with __refdata.)

Convert the driver to the more usual module_platform_driver().

This fixes a W=1 build warning:

	WARNING: modpost: drivers/char/hw_random/mxc-rnga: section mismatch in reference: mxc_rnga_driver+0x10 (section: .data) -> mxc_rnga_remove (section: .exit.text)

with CONFIG_HW_RANDOM_MXC_RNGA=m.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>