ARM: at91/dt: remove old clk material

This patch removes the old main clk node which is now useless as sama5d3
SoCs and boards are no longer compatible with the old at91 clk
implementations.

It also remove old clock definitions (clock definitions using at91 old clk
framework).

Signed-off-by: Boris BREZILLON <b.brezillon@overkiz.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>