Pin number, as specified in OF pinumx entries, is relative to each bank
start, whereas both at91_mux_pin() and at91_pinctrl_set_conf() asssume
absolute pin numbering, so we need to take into account each bank's pin
base and convert pin number appropriately. Failing to do so results in
any pinmux configuration being applied to pins in bank A.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>