The serial driver subsystem does not need any extra data, so it can use
`struct fdt_driver` directly. The generic fdt_serial_init() first
attempts to match the chosen stdout device, and upon failure matches the
first available serial device in the DT. It is a fatal error if no such
device is found. This matches the behavior of fdt_driver_init_one().
Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
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