efi_rng_protocol: Install the efi_rng_protocol on the root node

Install the EFI_RNG_PROTOCOL implementation for it's subsequent use by
the kernel for features like kaslr.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>