ARM: at91/dt: drop deprecated mainck node from pmc

The clock-frequency property became obsolete since the rework of the main
clock driver in 3.16 (see commit 27cb1c20

).

It now get and uses the clock-frequency from the main_xtal node.

Signed-off-by: Gaël PORTAY <g.portay@overkiz.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>