ARM: bcm283x: Define UART pinmuxing on board level

Until RPI 3 and Zero W the pl011 (uart0) was always on pin 14/15. So in
order to take care of them and other boards in the future,
we need to define UART pinmuxing on board level.

This work based on Eric Anholt's patch "ARM: bcm2385: Don't force pl011
onto pins 14/15." and Fabian Vogt's patch "ARM64: dts: bcm2837: assign
uart0 to BT and uart1 to pin headers".

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Eric Anholt <eric@anholt.net>