efi: qemu: arm64: Add efi_rng_protocol implementation for the platform

Add support for the EFI_RNG_PROTOCOL routines for the qemu arm64
platform. EFI_RNG_PROTOCOL is an uefi boottime service which is
invoked by the efi stub in the kernel for getting random seed for
kaslr.

The routines are platform specific, and use the virtio-rng device on
the platform to get random data.

The feature can be enabled through the following config
CONFIG_EFI_RNG_PROTOCOL

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Changed SPDX header to use /* instead of //.
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>