Documentation: DT: Renamed of-serial.txt to 8250.txt

The file of-serial.txt was only for 8250 compatible UART implementations,
so renamed it to 8250.txt to avoid confusing other persons.
This is suggested by Arnd, see:
http://lists.infradead.org/pipermail/linux-arm-kernel/2014-September/291455.htmlSigned-off-by: Chunyan Zhang <chunyan.zhang@spreadtrum.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>