dt-bindings: rng: clocks property on omap_rng not always mandatory

Commit 52060836 ("dt-bindings: omap-rng: Document SafeXcel IP-76
device variant") update the omap_rng Device Tree binding to add support
for the IP-76 variation of the IP. As part of this change, a "clocks"
property was added, but is indicated as "Required", without indicated
it's actually only required for some compatible strings.

This commit fixes that, by explicitly stating that the clocks property
is only required with the inside-secure,safexcel-eip76 compatible
string.

Fixes: 52060836 ("dt-bindings: omap-rng: Document SafeXcel IP-76 device variant")
Cc: <stable@vger.kernel.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>