memory: renesas-rpc-if: fix possible NULL pointer dereference of resource

The platform_get_resource_byname() can return NULL which would be
immediately dereferenced by resource_size().  Instead dereference it
after validating the resource.

Addresses-Coverity: Dereference null return value
Fixes: ca7d8b98 ("memory: add Renesas RPC-IF driver")
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Link: https://lore.kernel.org/r/20210407154357.70200-1-krzysztof.kozlowski@canonical.com