ARM64: dts: marvell: armada-37xx: Fix the number of GPIO on south bridge

The number of pins in South Bridge is 30 and not 29. There is a fix for
the driver for the pinctrl, but a fix is also need at device tree level
for the GPIO.

Fixes: afda007f

 ("ARM64: dts: marvell: Add pinctrl nodes for Armada
3700")
Cc: <stable@vger.kernel.org>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>