ARM: mvebu: make DTS comments reflect DEBUG_LL changes

The Kconfig symbol DEBUG_MVEBU_UART_ALTERNATE was renamed to
DEBUG_MVEBU_UART0_ALTERNATE. And the symbol DEBUG_MVEBU_UART1_ALTERNATE
was added to allow UART1 as a DEBUG_LL target. Make the comment at the
top of this DTS reflect those changes.

Since we're touching this DTS add comments to show which blocks describe
UART0 and UART1.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>