fdt: fix fdtdec_setup_memory_banksize()

Prior to this patch is fdtdec_setup_memory_banksize() incorrectly
ignoring the "status" field. This patch fixes that by testing the status
with fdtdec_get_is_enabled() before using a memory node.

Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>