Blackfin: move on-chip UART resources to boards files

Rather than keeping the pins in the actual driver and worrying about a
mess of Kconfig options, declare all the desired pin resources in the
boards file.  This lets people easily select the specific pins/ports for
the normal UART as well as GPIOs for CTS/RTS.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>