ion: Simplify carveout heap base/size retrieval/error handling

Use of_address_to_resource() to get the carveout base and size
in one shot. This also simplifies the error handling by allowing
us to decrement the reference count on the of_node in one place,
as opposed to two places.

Change-Id: I73cbc59fa4732a40308bf47a975c2cbbd6f8dfc2
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>