ARM OMAP2+ GPMC: calculate GPMCFCLKDIVIDER based on WAITMONITORINGTIME

The WAITMONITORINGTIME is expressed as a number of GPMC_CLK clock cycles,
even though the access is defined as asynchronous, and no GPMC_CLK clock
is provided to the external device. Still, GPMCFCLKDIVIDER is used as a divider
for the GPMC clock, so it must be programmed to define the
correct WAITMONITORINGTIME delay.

Calculate GPMCFCLKDIVIDER independent of gpmc,sync-clk-ps in DT for
pure asynchronous accesses, i.e. both read and write asynchronous.

Signed-off-by: Robert ABEL <rabel@cit-ec.uni-bielefeld.de>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Roger Quadros <rogerq@ti.com>