ARM: at91: remove all !DT related configuration options

OLD_CLK_AT91 & OLD_IRQ_AT91 were only selected by entries in Kconfig.non_dt
that are now gone. So we remove all this legacy stuff and select the proper
options in the SOC_ entries.

As USE_OF is now selected directly in arch/arm/Kconfig AT91 entry, we can
safely remove it everywhere in this file.

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