mx31: provide readable WEIM CS accessor

setup_weimcs() and some macros are added to support the setup
for i.MX31 WEIM chip selects. As a compromise between verbosity
and readability an ASCII-art'ish bit comment is used instead of
bitfields.
All i.MX31 boards have been patched to use this approach using a
helper program to verify the changes.

Signed-off-by: Helmut Raiger <helmut.raiger@hale.at>
Acked-by: Stefano Babic <sbabic@denx.de>