ARM: mvebu: Add UART1 as DEBUG_LL possible target

Some mvebu boards have the UART1 more easily accessible than the other UARTs
found on the system.

Add a debug_ll option for this case.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>