ARM: at91: remove unused CONFIG_ARCH_AT91SAM9G45 option

The CONFIG_ARCH_AT91SAM9G45 config option was removed by ("ARM: at91: remove
at91sam9g45/9m10 legacy board support") so cleanup the use of it.

Reported-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>