ARM: dts: Add UART skip-init properties for U-boot

U-boot can get stuck trying to initialise UARTs that aren't mapped
to the pin header. There is no reason for U-boot not to rely on the
initialisation by the firmware, so tag both UARTs with the u-boot
magic boolean property "skip-init".

See: https://github.com/raspberrypi/linux/pull/3731
     https://lists.denx.de/pipermail/u-boot/2017-April/285606.html



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