efi_loader: enable RNG if DM_RNG is enabled

Enable EFI_RNG_PROTOCOL by default if DM_RNG is set.

Signed-off-by: Peter Robinson <pbrobinson@gmail.com>
Acked-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Remove superfluous 'if DM_RNG'
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>