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.html

Signed-off-by: Chunyan Zhang <[e-mail]>
Acked-by: Mark Rutland <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>