sandbox: dtsi: add rng

sandbox: dtsi: add rng

Having an rng in the sandbox is useful not only for tests but also for e.g.
UEFI. Therefore, copy the rng node from test.dts to sandbox.dtsi.

In the case of UEFI, it can then be verified with `efidebug dh' that a
"Random Number Generator" protocol is indeed present.

This also fixes the following `bootefi' error:

  Missing RNG device for EFI_RNG_PROTOCOL

Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>
Cc: Simon Glass <sjg@chromium.org>