ARM: dts: at91/trivial: remove old NAND bindings leftover in sama5d2

As the new bindings are already in place in sama5d2.dtsi and that it's used
in the only Mainline board for this product (at91-sama5d2_ptc_ek.dts), we can
safely remove the old bindings.

Cc: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>