dt-bindings: chosen: document kaslr-seed property

Document the /chosen/kaslr-seed property (and its interaction with the
EFI_RNG_PROTOCOL API). Thanks to Ard for clarifications.

Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>