blackfin: make name_to_gpio be a weak symbol

This required moving it into a C file from the header.

The only user of a non-default name_to_gpio is blackfin, therefore build tested
with the blackfin bct-brettl2 build, which is one I picked at random. Also
tested with a build for the ARM tec board which uses the default/fallback
implementation. Inspection with objdump shows that both have done the right
thing.

This change was requested by Marek during review of the sunxi patch series.

Signed-off-by: Ian Campbell <ijc@hellion.org.uk>
Cc: Marek Vasut <marex@denx.de>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Sonic Zhang <sonic.adi@gmail.com>