ARM: at91/at91-pinctrl documentation: fix typo and add some details

The relation between PIN_BANK numbers and pio letters wasn't made very
clear.

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