uart_prop scan only checks first compatible item

[tmagik]

If a device tree (such as the new linux one at https://github.com/sifive/riscv-linux/blob/dev/paulw/test-upstream-v4.20-rc7-for-troy/arch/riscv/boot/dts/sifive/fu540-c000.dtsi#L156 ) puts another 'compatible' string, the code does not check any other strings and fails to find the serial port.