ARM: at91: factorise duplicated at91sam9 idle

Remove duplicated at91sam9xxxx_idle() functions introduced
by commit c9dfafba "ARM: mach-at91: move special idle code out of line".
Replace by a generic at91sam9_idle() function in setup.c common
location.

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>