ARM: dts: at91: sama5d3: switch to new clock bindings

Switch sama5d3 boards to the new PMC clock bindings.

This prevents the wb50n to use the out of spec rate for USART1.

Link: https://lore.kernel.org/r/20200110222744.1261464-1-alexandre.belloni@bootlin.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>