ARM: dts: bcm27xx: Add stdout-path to serial0

Rather than deleting the upstream stdout-path declaration, overwrite
it with one selecting serial0, which will always be the UART mapped
to the 40-pin header (provided enable_uart=1 is specified). Doing
so has the advantage that earlycon can be configured just by adding
"earlycon" to the command line.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>