pinctrl: dt-bindings: Fix A37xx uart2 group name

Fix a typo in A37xx pin controllers documentation about uart2 pin group.

Signed-off-by: Miquel Raynal <miquel.raynal@free-electrons.com>
Reviewed-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>