raspberrypi, raspberrypi2: document the two consoles (UART+HDMI)

raspberrypi, raspberrypi2: document the two consoles (UART+HDMI)

The defconfigs for the Raspberry Pi boards start a console on HDMI in
addition to the classic one on UART. Document this feature in the
readme.

Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>