ARM: at91/gpio: change comments and one variable name

What was true only on at91sam9263 about the sharing of a single AIC
IRQ line for several GPIO banks is now used by several Atmel SoCs.

Change a variable name to allow better understanding while
introducing IRQ domains in following patches.

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