crypto/fsl: Introduce SPL_FSL_CAAM_RNG

Add SPL variant of SPL_FSL_CAAM_RNG so that the SPL_FSL_CAAM_RNG can
be disabled in SPL if necessary. This may be necessary due to e.g.
size constraints of the SPL.

Signed-off-by: Marek Vasut <marex@denx.de>