omap_gpmc: Refactor omap_gpmc_cs_map and omap_gpmc_cs_unmap

Refactor the omap_gpmc_cs_map/unmap functions:
 * take the omap_gpmc_s* and a chipselect id rather than the
   omap_gpmc_cs_file_s*, so they have access to the general gpmc
   member fields
 * extract the base and mask from the config registers in the functions
   rather than at every callsite
 * check for CSVALID in the functions rather than at every callsite

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
