ARM: at91: finally drop at91_sys_read/write

Remove at91_sys_read/write() from io.h file. This function
is not used anymore and was a stopper on the way to single
zImage kernel for AT91.

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