GPMC updates:
 - bugfixes: wrong/missing flags, omitted write, wrong test
 - don't map memory segments starting at zero
 - improve debug messaging
 - export gpmc_get_fclk_perio]d() since it's needed to calc timings
 - expect gpmc_cs_set_timings() caller to have initialized sync vs async

Note that this API is glitchy; likely the best fix would be to add
a member to "struct gpmc_timings" to hold GPMC_CONFIG1, since that
holds one key aspect of the GPMC timings (the gpmc_fclk divisor,
and sync vs. async == whether that divisor matters).
Signed-off-by: NDavid Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: NTony Lindgren <tony@atomide.com>