ARM: mvebu: Rename DEBUG_LL to indicate UART index

The mvebu SoCs actually have more UARTs than just the one exposed in DEBUG_LL
yet.

In order to differentiate them, Add the index in the configuration options and
their help.

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>