media: dt-bindings: nokia,smia: Make vana-supply optional

vana-supply is optional in the spec, therefore make it optional in
bindings, too.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>