ARM: at91/dt: fix macb pinctrl_macb_rmii_mii_alt definition

The PA24 pin is wrongly assigned to peripheral B.
In the current config there is 2 ETX3 pins (PA11 and PA24) and
no ETXER pin (PA22).

Signed-off-by: Boris BREZILLON <b.brezillon@overkiz.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: stable <stable@vger.kernel.org> # 3.8+