Allow AM33xx boards to setup GPMC chipselects.

Expose the enable_gpmc_cs_config() function so AM33xx based boards can register GPMC chip selects.

Changes in V4:
- Fix checkpatch errors (TAB -> space mangling)

Changes in V3:
- Fix line wrapping

Changes in V2:
- Indicate this is for AM33xx (not OMAP2)

Signed-off-by: Mark Jackson <mpfj@newflow.co.uk>
Acked-by: Peter Korsgaard <jacmet@sunsite.dk>