arm64: dts: marvell: Fix A37xx UART0 register size

Armada-37xx UART0 registers are 0x200 bytes wide. Right next to them are
the UART1 registers that should not be declared in this node.

Update the example in DT bindings document accordingly.

Signed-off-by: allen yan <yanwei@marvell.com>
Signed-off-by: Miquel Raynal <miquel.raynal@free-electrons.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>