mmc: mtk-sd: check for valid optional memory resource

'top_base' memory region is optional. Check that the resource is valid
before using it. This avoid getting a "invalid resource" error message
printed by the kernel.

Signed-off-by: Fabien Parent <fparent@baylibre.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>