arch: toolchain: Introduce target CPU revision.

Adds the possibility to have a free-form CPU revision string and append it
to the target CPU. Only Blackfin actually uses this option.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>