ARM: at91: remove old at91-specific clock driver

This clock driver collection was specific to AT91 and only used in !DT cases.
All clocks and the clock trees for all Atmel SoCs are now described by drivers
using the common clock framework.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Boris BREZILLON <boris.brezillon@free-electrons.com>