serial:blackfin: rename Blackfin serial driver to bfin_uart.c

bfin_5xx.c is not a general name for all Blackfin chips.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>