ARM: dts: at91: sama5d2: set the sdmmc gclk frequency

Set the frequency of the generated clock used by sdmmc devices in order
to not rely on the configuration done by previous components.

Signed-off-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Link: https://lore.kernel.org/r/20191128074522.69706-3-ludovic.desroches@microchip.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>