MAINTAINERS: Remove Piotr Sroka and mark Cadence NFC as orphaned

Piotr's address is bouncing, remove him from MAINTAINERS and mark the
driver he was maintaining, Cadence's, as orphaned.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200510211809.15610-1-miquel.raynal@bootlin.com