The Raspberry Pi has two different UART devices pin-muxed to GPIO 14&15:
One ARM PL011 one and the 8250 compatible "Mini-UART".
A dtoverlay parameter in config.txt will tell the firmware to switch
between the two: it will setup the right clocks and will configure the
pinmuxes accordingly.

To autodetect the user's choice, we read the pinmux register and check
its setting: ALT5 (0x2) means the Mini-UART is used, ALT0 (0x4) points
to the PL011.
Based on that we select the UART driver to initialise.

This will allow console output in any case.

Change-Id: I620d3ce68de6c6576599f2a405636020e1fd1376
Signed-off-by: Andre Przywara <andre.przywara@arm.com>