mtd: lpddr2_nvm: Fix possible null-ptr-deref

It will cause null-ptr-deref when resource_size(add_range) invoked,
if platform_get_resource() returns NULL.

Fixes: 96ba9dd6 ("mtd: lpddr: add driver for LPDDR2-NVM PCM memories")
Signed-off-by: Hui Tang <tanghui20@huawei.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20221114090240.244172-1-tanghui20@huawei.com