ARM: dts: at91: sama5d2: use the divided clock for SMC

The SAMA5D2 is different from SAMA5D3 and SAMA5D4, as there are two
different clocks for the peripherals in the SoC. The Static Memory
controller is connected to the divided master clock.

Unfortunately, the device tree does not correctly show this and uses the
master clock directly. This clock is then used by the code for the NAND
controller to calculate the timings for the controller, and we end up with
slow NAND Flash access.

Fix the device tree, and the performance of Flash access is improved.

Signed-off-by: Romain Izard <romain.izard.pro@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>